| Message ID | 1452578622-4492-1-git-send-email-den@openvz.org |
|---|---|
| State | New |
| Headers | show |
Am 12.01.2016 um 07:03 hat Denis V. Lunev geschrieben: > There is a long-long story. OVMF VMs can not be snapsotted using > 'virsh snapshot' as they have "pflash" device which is configured as > "raw" image. There was a discussion in the past about that. > > Good description has been provided on topic by Laszlo Ersek, see below: > > "It is true that a pflash drive is "just a drive" *internally* to QEMU. > It is also true that it more or less takes the same -drive options as > any other *disk* drive. But those facts are just implementation details. > > The relevant trait of pflash storage files is that they are not *disk > images*, on the libvirt domain XML level. They are not created in > storage pools, you cannot specify their caching attributes, you don't > specify their guest-visible frontend in separation (like virtio-blk / > virtio-scsi / pflash). Those details are hidden (on purpose). > > Consequently, pflash storage files are expected to be *small* in size > (in practice: identically sized to the varstore template they are > instantiated from). They are created under /var/lib/libvirt/qemu/nvram. > Although you can edit their path in the domain XML, they are not > considered disks." > > Thus we should avoid selection of "pflash" drives for VM state saving. > > For now "pflash" is read-write raw image as it configured by libvirt. > Thus there are no such images in the field and we could safely disable > ability to save state to those images inside QEMU. This is obviously broken. If you write to the pflash, then it needs to be snapshotted in order to keep a consistent state. If you want to avoid snapshotting the image, make it read-only and it will be skipped even today. Kevin
On 12/01/2016 15:16, Kevin Wolf wrote: >> Thus we should avoid selection of "pflash" drives for VM state saving. >> >> For now "pflash" is read-write raw image as it configured by libvirt. >> Thus there are no such images in the field and we could safely disable >> ability to save state to those images inside QEMU. > > This is obviously broken. If you write to the pflash, then it needs to > be snapshotted in order to keep a consistent state. > > If you want to avoid snapshotting the image, make it read-only and it > will be skipped even today. Sort of. The point of having flash is to _not_ make it read-only, so that is not a solution. Flash is already being snapshotted as part of saving RAM state. In fact, for this reason the device (at least the one used with OVMF; I haven't checked other pflash devices) can simply save it back to disk on the migration destination, without the need to use "migrate -b" or shared storage. See commit 4c0cfc72b31a79f737a64ebbe0411e4b83e25771: Author: Laszlo Ersek <lersek@redhat.com> Date: Sat Aug 23 12:19:07 2014 +0200 pflash_cfi01: write flash contents to bdrv on incoming migration A drive that backs a pflash device is special: - it is very small, - its entire contents are kept in a RAMBlock at all times, covering the guest-phys address range that provides the guest's view of the emulated flash chip. The pflash device model keeps the drive (the host-side file) and the guest-visible flash contents in sync. When migrating the guest, the guest-visible flash contents (the RAMBlock) is migrated by default, but on the target host, the drive (the host-side file) remains in full sync with the RAMBlock only if: - the source and target hosts share the storage underlying the pflash drive, - or the migration requests full or incremental block migration too, which then covers all drives. Due to the special nature of pflash drives, the following scenario makes sense as well: - no full nor incremental block migration, covering all drives, alongside the base migration (justified eg. by shared storage for "normal" (big) drives), - non-shared storage for pflash drives. In this case, currently only those portions of the flash drive are updated on the target disk that the guest reprograms while running on the target host. In order to restore accord, dump the entire flash contents to the bdrv in a post_load() callback. - The read-only check follows the other call-sites of pflash_update(); - both "pfl->ro" and pflash_update() reflect / consider the case when "pfl->bs" is NULL; - the total size of the flash device is calculated as in pflash_cfi01_realize(). When using shared storage, or requesting full or incremental block migration along with the normal migration, the patch should incur a harmless rewrite from the target side. It is assumed that, on the target host, RAM is loaded ahead of the call to pflash_post_load(). I don't like very much using IF_PFLASH this way, which is why I hadn't replied to the patch so far---I hadn't made up my mind about *what* to suggest instead, or whether to just accept it. However, it does work. Perhaps a separate "I know what I am doing" skip-snapshot option? Or a device callback saying "not snapshotting this is fine"? Paolo
On 01/12/2016 05:16 PM, Kevin Wolf wrote: > Am 12.01.2016 um 07:03 hat Denis V. Lunev geschrieben: >> There is a long-long story. OVMF VMs can not be snapsotted using >> 'virsh snapshot' as they have "pflash" device which is configured as >> "raw" image. There was a discussion in the past about that. >> >> Good description has been provided on topic by Laszlo Ersek, see below: >> >> "It is true that a pflash drive is "just a drive" *internally* to QEMU. >> It is also true that it more or less takes the same -drive options as >> any other *disk* drive. But those facts are just implementation details. >> >> The relevant trait of pflash storage files is that they are not *disk >> images*, on the libvirt domain XML level. They are not created in >> storage pools, you cannot specify their caching attributes, you don't >> specify their guest-visible frontend in separation (like virtio-blk / >> virtio-scsi / pflash). Those details are hidden (on purpose). >> >> Consequently, pflash storage files are expected to be *small* in size >> (in practice: identically sized to the varstore template they are >> instantiated from). They are created under /var/lib/libvirt/qemu/nvram. >> Although you can edit their path in the domain XML, they are not >> considered disks." >> >> Thus we should avoid selection of "pflash" drives for VM state saving. >> >> For now "pflash" is read-write raw image as it configured by libvirt. >> Thus there are no such images in the field and we could safely disable >> ability to save state to those images inside QEMU. > This is obviously broken. If you write to the pflash, then it needs to > be snapshotted in order to keep a consistent state. > > If you want to avoid snapshotting the image, make it read-only and it > will be skipped even today. > > Kevin you interpret the patch a bit wrong. It will be snapshoted once I'll raw image with qcow2 image, but this image will not be selected for state saving, i.e. it will remain compact. Den
On 01/12/2016 05:59 PM, Paolo Bonzini wrote: > > On 12/01/2016 15:16, Kevin Wolf wrote: >>> Thus we should avoid selection of "pflash" drives for VM state saving. >>> >>> For now "pflash" is read-write raw image as it configured by libvirt. >>> Thus there are no such images in the field and we could safely disable >>> ability to save state to those images inside QEMU. >> This is obviously broken. If you write to the pflash, then it needs to >> be snapshotted in order to keep a consistent state. >> >> If you want to avoid snapshotting the image, make it read-only and it >> will be skipped even today. > Sort of. The point of having flash is to _not_ make it read-only, so > that is not a solution. > > Flash is already being snapshotted as part of saving RAM state. In > fact, for this reason the device (at least the one used with OVMF; I > haven't checked other pflash devices) can simply save it back to disk > on the migration destination, without the need to use "migrate -b" or > shared storage. > > See commit 4c0cfc72b31a79f737a64ebbe0411e4b83e25771: > > Author: Laszlo Ersek <lersek@redhat.com> > Date: Sat Aug 23 12:19:07 2014 +0200 > > pflash_cfi01: write flash contents to bdrv on incoming migration > > A drive that backs a pflash device is special: > - it is very small, > - its entire contents are kept in a RAMBlock at all times, covering the > guest-phys address range that provides the guest's view of the emulated > flash chip. > > The pflash device model keeps the drive (the host-side file) and the > guest-visible flash contents in sync. When migrating the guest, the > guest-visible flash contents (the RAMBlock) is migrated by default, but on > the target host, the drive (the host-side file) remains in full sync with > the RAMBlock only if: > - the source and target hosts share the storage underlying the pflash > drive, > - or the migration requests full or incremental block migration too, which > then covers all drives. > > Due to the special nature of pflash drives, the following scenario makes > sense as well: > - no full nor incremental block migration, covering all drives, alongside > the base migration (justified eg. by shared storage for "normal" (big) > drives), > - non-shared storage for pflash drives. > > In this case, currently only those portions of the flash drive are updated > on the target disk that the guest reprograms while running on the target > host. > > In order to restore accord, dump the entire flash contents to the bdrv in > a post_load() callback. > > - The read-only check follows the other call-sites of pflash_update(); > - both "pfl->ro" and pflash_update() reflect / consider the case when > "pfl->bs" is NULL; > - the total size of the flash device is calculated as in > pflash_cfi01_realize(). > > When using shared storage, or requesting full or incremental block > migration along with the normal migration, the patch should incur a > harmless rewrite from the target side. > > It is assumed that, on the target host, RAM is loaded ahead of the call to > pflash_post_load(). > > I don't like very much using IF_PFLASH this way, which is why I hadn't > replied to the patch so far---I hadn't made up my mind about *what* to > suggest instead, or whether to just accept it. However, it does work. > > Perhaps a separate "I know what I am doing" skip-snapshot option? Or > a device callback saying "not snapshotting this is fine"? > > Paolo Paolo, it looks I have made a bad description :( The idea of this patch was trivial. First of all, I would like to keep this image internally snapshoted. That is why the ultimate goal was to switch from raw to qcow2 to keep changes inside the image. Though in this case this drive could be selected to save VM state, which could be big. The function being changed selects the image for VM state saving. here I would like to skip IP_PFLASH from being selected to keep it small as required by libvirt guys. Den
On 12 January 2016 at 15:13, Denis V. Lunev <den@openvz.org> wrote: > The idea of this patch was trivial. First of all, I would like to keep > this image internally snapshoted. That is why the ultimate goal > was to switch from raw to qcow2 to keep changes inside the > image. > > Though in this case this drive could be selected to save VM > state, which could be big. The function being changed selects > the image for VM state saving. > > here I would like to skip IP_PFLASH from being selected to keep > it small as required by libvirt guys. This has to be a board specific decision. Some of our machine models might have no backing storage other than an IP_PFLASH drive, but it's still nice to be able to do vmsave/vmload on them. thanks -- PMM
Am 12.01.2016 um 15:59 hat Paolo Bonzini geschrieben: > > > On 12/01/2016 15:16, Kevin Wolf wrote: > >> Thus we should avoid selection of "pflash" drives for VM state saving. > >> > >> For now "pflash" is read-write raw image as it configured by libvirt. > >> Thus there are no such images in the field and we could safely disable > >> ability to save state to those images inside QEMU. > > > > This is obviously broken. If you write to the pflash, then it needs to > > be snapshotted in order to keep a consistent state. > > > > If you want to avoid snapshotting the image, make it read-only and it > > will be skipped even today. > > Sort of. The point of having flash is to _not_ make it read-only, so > that is not a solution. > > Flash is already being snapshotted as part of saving RAM state. In > fact, for this reason the device (at least the one used with OVMF; I > haven't checked other pflash devices) can simply save it back to disk > on the migration destination, without the need to use "migrate -b" or > shared storage. > [...] > I don't like very much using IF_PFLASH this way, which is why I hadn't > replied to the patch so far---I hadn't made up my mind about *what* to > suggest instead, or whether to just accept it. However, it does work. > > Perhaps a separate "I know what I am doing" skip-snapshot option? Or > a device callback saying "not snapshotting this is fine"? Boy, is this ugly... What do you do with disk-only snapshots? The recovery only works as long as you have VM state. Kevin
Am 12.01.2016 um 16:16 hat Peter Maydell geschrieben: > On 12 January 2016 at 15:13, Denis V. Lunev <den@openvz.org> wrote: > > The idea of this patch was trivial. First of all, I would like to keep > > this image internally snapshoted. That is why the ultimate goal > > was to switch from raw to qcow2 to keep changes inside the > > image. > > > > Though in this case this drive could be selected to save VM > > state, which could be big. The function being changed selects > > the image for VM state saving. > > > > here I would like to skip IP_PFLASH from being selected to keep > > it small as required by libvirt guys. > > This has to be a board specific decision. Some of our machine > models might have no backing storage other than an IP_PFLASH > drive, but it's still nice to be able to do vmsave/vmload on them. Maybe we can give flash images lower priority than other images? I'm not sure if we don't break compatibility with such a change, though. loadvm on existing snapshots could fail now. We might need to change that first so that it can find snapshots even on images that wouldn't be the VM state image for new snapshots any more. Kevin
Am 12.01.2016 um 16:10 hat Denis V. Lunev geschrieben: > On 01/12/2016 05:16 PM, Kevin Wolf wrote: > >Am 12.01.2016 um 07:03 hat Denis V. Lunev geschrieben: > >>There is a long-long story. OVMF VMs can not be snapsotted using > >>'virsh snapshot' as they have "pflash" device which is configured as > >>"raw" image. There was a discussion in the past about that. > >> > >>Good description has been provided on topic by Laszlo Ersek, see below: > >> > >>"It is true that a pflash drive is "just a drive" *internally* to QEMU. > >>It is also true that it more or less takes the same -drive options as > >>any other *disk* drive. But those facts are just implementation details. > >> > >>The relevant trait of pflash storage files is that they are not *disk > >>images*, on the libvirt domain XML level. They are not created in > >>storage pools, you cannot specify their caching attributes, you don't > >>specify their guest-visible frontend in separation (like virtio-blk / > >> virtio-scsi / pflash). Those details are hidden (on purpose). > >> > >>Consequently, pflash storage files are expected to be *small* in size > >>(in practice: identically sized to the varstore template they are > >>instantiated from). They are created under /var/lib/libvirt/qemu/nvram. > >>Although you can edit their path in the domain XML, they are not > >>considered disks." > >> > >>Thus we should avoid selection of "pflash" drives for VM state saving. > >> > >>For now "pflash" is read-write raw image as it configured by libvirt. > >>Thus there are no such images in the field and we could safely disable > >>ability to save state to those images inside QEMU. > >This is obviously broken. If you write to the pflash, then it needs to > >be snapshotted in order to keep a consistent state. > > > >If you want to avoid snapshotting the image, make it read-only and it > >will be skipped even today. > > > >Kevin > you interpret the patch a bit wrong. > > It will be snapshoted once I'll raw image with qcow2 image, but this image > will not be selected for state saving, i.e. it will remain compact. Sorry, I misunderstood. That's more reasonable indeed. Kevin
On 12/01/2016 16:20, Kevin Wolf wrote: > > Flash is already being snapshotted as part of saving RAM state. In > > fact, for this reason the device (at least the one used with OVMF; I > > haven't checked other pflash devices) can simply save it back to disk > > on the migration destination, without the need to use "migrate -b" or > > shared storage. > > Boy, is this ugly... > > What do you do with disk-only snapshots? The recovery only works as long > as you have VM state. Turns out I had misunderstood Denis's patch, but FWIW this _is_ done as part of migration or savevm, so the VM state is available. Paolo
On 01/12/2016 06:20 PM, Kevin Wolf wrote: > Am 12.01.2016 um 15:59 hat Paolo Bonzini geschrieben: >> >> On 12/01/2016 15:16, Kevin Wolf wrote: >>>> Thus we should avoid selection of "pflash" drives for VM state saving. >>>> >>>> For now "pflash" is read-write raw image as it configured by libvirt. >>>> Thus there are no such images in the field and we could safely disable >>>> ability to save state to those images inside QEMU. >>> This is obviously broken. If you write to the pflash, then it needs to >>> be snapshotted in order to keep a consistent state. >>> >>> If you want to avoid snapshotting the image, make it read-only and it >>> will be skipped even today. >> Sort of. The point of having flash is to _not_ make it read-only, so >> that is not a solution. >> >> Flash is already being snapshotted as part of saving RAM state. In >> fact, for this reason the device (at least the one used with OVMF; I >> haven't checked other pflash devices) can simply save it back to disk >> on the migration destination, without the need to use "migrate -b" or >> shared storage. >> [...] >> I don't like very much using IF_PFLASH this way, which is why I hadn't >> replied to the patch so far---I hadn't made up my mind about *what* to >> suggest instead, or whether to just accept it. However, it does work. >> >> Perhaps a separate "I know what I am doing" skip-snapshot option? Or >> a device callback saying "not snapshotting this is fine"? > Boy, is this ugly... > > What do you do with disk-only snapshots? The recovery only works as long > as you have VM state. > > Kevin actually I am in a bit of trouble :( I understand that this is ugly, but I would like to make working 'virsh snapshot' for OVFM VMs. This is necessary for us to make a release. Currently libvirt guys generate XML in the following way: <os> <type arch='x86_64' machine='pc-i440fx-2.3'>hvm</type> <loader readonly='yes' type='pflash'>/usr/share/OVMF/OVMF_CODE_new.fd</loader> <nvram>/var/lib/libvirt/qemu/nvram/f20efi_VARS.fd</nvram> </os> This results in: qemu -drive file=/usr/share/OVMF/OVMF_CODE_new.fd,if=pflash,format=raw,unit=0,readonly=on \ -drive file=/var/lib/libvirt/qemu/nvram/f20efi_VARS.fd,if=pflash,format=raw,unit=1 This obviously can not pass check in bdrv_all_can_snapshot() as 'pflash' is RW and raw, i.e. can not be snapshoted. They have discussed the switch to the following command line: qemu -drive file=/usr/share/OVMF/OVMF_CODE_new.fd,if=pflash,format=raw,unit=0,readonly=on \ -drive file=/var/lib/libvirt/qemu/nvram/f20efi_VARS.fd.qcow2,if=pflash,format=qcow2,unit=1 and say that in this case VM state could fall into PFLASH drive which is should not be big as the location of the file is different. This means that I am doomed here. Either we should force libvirt people to forget about their opinion that pflash should be small which I am unable to do or I should invent a way to ban VM state saving into pflash. OK. There are 2 options. 1) Ban pflash as it was done. 2) Add 'no-vmstate' flag to -drive (invented just now). Den P.S. Here is a summary that my colleague has receiver from libvirt list. -------- Forwarded Message -------- Subject: Re: Snapshotting OVMF guests Date: Mon, 11 Jan 2016 13:56:29 +0100 From: Laszlo Ersek <lersek@redhat.com> To: Dmitry Andreev <dandreev@virtuozzo.com> CC: Michal Privoznik <mprivozn@redhat.com>, Markus Armbruster <armbru@redhat.com> Hello Dmitry, (Cc: Markus.) > https://bugzilla.redhat.com/show_bug.cgi?id=1180955 I have now re-read that BZ. In comment 7 I wrote, > However, if Michal's v2 libvirt patchset was applied, and the varstore > drive was qcow2, then qemu would dump the *entire VM state*, including > memory and device state, into the varstore drive (the 6th drive) under > the command line visible in comment #0. That's *completely* bogus; > much worse than rejecting the snapshot request. It is bogus for size and configuration reasons. It is true that a pflash drive is "just a drive" *internally* to QEMU. It is also true that it more or less takes the same -drive options as any other *disk* drive. But those facts are just implementation details. The relevant trait of pflash storage files is that they are not *disk images*, on the libvirt domain XML level. They are not created in storage pools, you cannot specify their caching attributes, you don't specify their guest-visible frontend in separation (like virtio-blk / virtio-scsi / pflash). Those details are hidden (on purpose). Consequently, pflash storage files are expected to be *small* in size (in practice: identically sized to the varstore template they are instantiated from). They are created under /var/lib/libvirt/qemu/nvram. Although you can edit their path in the domain XML, they are not considered disks. This is also reflected in the way they are migrated. They are not migrated with NBD / live storage migration / blockdev migration. Instead, on the target host, when the in-migration completes, the entire contents of the flash drive are written out in one shot to the target host file. Please see: - the pflash_post_load() function in QEMU's "hw/block/pflash_cfi01.c", - and QEMU commit 4c0cfc72. Storing large amounts or data in the pflash storage file would be incompatible with this concept. ... We also had an internal team discussion at Red Hat about this. I won't re-read it now, but I think I can share a part of my own BZ comment 9. In that comment I tried to summarize the internal discussion more or less for myself. (I made that comment private because it contained RH product related bits too -- I won't quote those bits now.) So from comment 9: > [...] the upshot from [the internal discussion] seems to be that > "savevm" is *in general* inappropriate for any non-trivial -drive > setup and/or for a -drive setup that is subject to change (eg. > reordering on the command line). Comment 11 in the BZ shows that we plan to document the limitation that internal snapshotting will never be supported for OVMF. External snapshotting *should* be, but it isn't yet either (because, at least at the time of writing the BZ comment, reverting to external snapshots wasn't supported). Bottom line, pflash is implemented as a drive internally, but it is not considered a *disk* drive, for migration, snapshotting, being stored in pools, or for storing large amounts of data. I hope this helps. If you'd like to enable snapshotting for OVMF virtual machines, that would be awesome; but I think it would require implementing the above-mentioned "revert to external snapshot" functionality. Thanks! Laszlo
meta comment here: On 01/12/16 16:47, Denis V. Lunev wrote: > P.S. Here is a summary that my colleague has receiver from libvirt > list. > > -------- Forwarded Message -------- > Subject: Re: Snapshotting OVMF guests > Date: Mon, 11 Jan 2016 13:56:29 +0100 > From: Laszlo Ersek <lersek@redhat.com> > To: Dmitry Andreev <dandreev@virtuozzo.com> > CC: Michal Privoznik <mprivozn@redhat.com>, Markus Armbruster > <armbru@redhat.com> > > Hello Dmitry, > > [...] Your colleague Dmitry did not receive this from the libvirt list. He received the from me in private. See the headers above. Please do not publicize a private exchange without asking for permission first. In the present case I don't mind it. I stand by everything I said, and I would have written mostly the same if I had been contacted publicly, on-list. But if you contact me in private *first*, then I expect the discussion to remain private. If you want to forward the email to a public list, please ask for permission. Otherwise I might consider it more prudent for myself to answer all private queries with just "please ask me this on the list instead". I appreciate that you guys are working on this, but let's handle emails sensibly. Thanks Laszlo
On 01/13/2016 01:37 PM, Laszlo Ersek wrote: > meta comment here: > > On 01/12/16 16:47, Denis V. Lunev wrote: > >> P.S. Here is a summary that my colleague has receiver from libvirt >> list. >> >> -------- Forwarded Message -------- >> Subject: Re: Snapshotting OVMF guests >> Date: Mon, 11 Jan 2016 13:56:29 +0100 >> From: Laszlo Ersek <lersek@redhat.com> >> To: Dmitry Andreev <dandreev@virtuozzo.com> >> CC: Michal Privoznik <mprivozn@redhat.com>, Markus Armbruster >> <armbru@redhat.com> >> >> Hello Dmitry, >> >> [...] > Your colleague Dmitry did not receive this from the libvirt list. He > received the from me in private. See the headers above. > > Please do not publicize a private exchange without asking for permission > first. > > In the present case I don't mind it. I stand by everything I said, and I > would have written mostly the same if I had been contacted publicly, > on-list. > > But if you contact me in private *first*, then I expect the discussion > to remain private. If you want to forward the email to a public list, > please ask for permission. Otherwise I might consider it more prudent > for myself to answer all private queries with just "please ask me this > on the list instead". > > I appreciate that you guys are working on this, but let's handle emails > sensibly. > > Thanks > Laszlo > Sorry :( I have not properly checked the message :( I am guilty.. Den
On 01/13/16 12:11, Denis V. Lunev wrote: > On 01/13/2016 01:37 PM, Laszlo Ersek wrote: >> meta comment here: >> >> On 01/12/16 16:47, Denis V. Lunev wrote: >> >>> P.S. Here is a summary that my colleague has receiver from libvirt >>> list. >>> >>> -------- Forwarded Message -------- >>> Subject: Re: Snapshotting OVMF guests >>> Date: Mon, 11 Jan 2016 13:56:29 +0100 >>> From: Laszlo Ersek <lersek@redhat.com> >>> To: Dmitry Andreev <dandreev@virtuozzo.com> >>> CC: Michal Privoznik <mprivozn@redhat.com>, Markus Armbruster >>> <armbru@redhat.com> >>> >>> Hello Dmitry, >>> >>> [...] >> Your colleague Dmitry did not receive this from the libvirt list. He >> received the from me in private. See the headers above. >> >> Please do not publicize a private exchange without asking for permission >> first. >> >> In the present case I don't mind it. I stand by everything I said, and I >> would have written mostly the same if I had been contacted publicly, >> on-list. >> >> But if you contact me in private *first*, then I expect the discussion >> to remain private. If you want to forward the email to a public list, >> please ask for permission. Otherwise I might consider it more prudent >> for myself to answer all private queries with just "please ask me this >> on the list instead". >> >> I appreciate that you guys are working on this, but let's handle emails >> sensibly. >> >> Thanks >> Laszlo >> > Sorry :( I have not properly checked the message :( > > I am guilty.. No prob, it's just that I've burned myself a few times before, hence I've grown to double check the address list when receiving & sending email. "List address not present" implies "other guy wants it to be private" to me. :) Cheers Laszlo
On 01/12/2016 09:03 AM, Denis V. Lunev wrote: > There is a long-long story. OVMF VMs can not be snapsotted using > 'virsh snapshot' as they have "pflash" device which is configured as > "raw" image. There was a discussion in the past about that. results of the discussion are available in this submission: [PATCH v5 0/8] QMP wrappers for VM snapshot operations
diff --git a/block/snapshot.c b/block/snapshot.c index 2d86b88..1a03581 100644 --- a/block/snapshot.c +++ b/block/snapshot.c @@ -25,6 +25,7 @@ #include "block/snapshot.h" #include "block/block_int.h" #include "qapi/qmp/qerror.h" +#include "sysemu/blockdev.h" QemuOptsList internal_snapshot_opts = { .name = "snapshot", @@ -481,8 +482,14 @@ BlockDriverState *bdrv_all_find_vmstate_bs(void) BlockDriverState *bs = NULL; while (not_found && (bs = bdrv_next(bs))) { + DriveInfo *dinfo; AioContext *ctx = bdrv_get_aio_context(bs); + dinfo = bs->blk != NULL ? blk_legacy_dinfo(bs->blk) : NULL; + if (dinfo != NULL && dinfo->type == IF_PFLASH) { + continue; + } + aio_context_acquire(ctx); not_found = !bdrv_can_snapshot(bs); aio_context_release(ctx);
There is a long-long story. OVMF VMs can not be snapsotted using 'virsh snapshot' as they have "pflash" device which is configured as "raw" image. There was a discussion in the past about that. Good description has been provided on topic by Laszlo Ersek, see below: "It is true that a pflash drive is "just a drive" *internally* to QEMU. It is also true that it more or less takes the same -drive options as any other *disk* drive. But those facts are just implementation details. The relevant trait of pflash storage files is that they are not *disk images*, on the libvirt domain XML level. They are not created in storage pools, you cannot specify their caching attributes, you don't specify their guest-visible frontend in separation (like virtio-blk / virtio-scsi / pflash). Those details are hidden (on purpose). Consequently, pflash storage files are expected to be *small* in size (in practice: identically sized to the varstore template they are instantiated from). They are created under /var/lib/libvirt/qemu/nvram. Although you can edit their path in the domain XML, they are not considered disks." Thus we should avoid selection of "pflash" drives for VM state saving. For now "pflash" is read-write raw image as it configured by libvirt. Thus there are no such images in the field and we could safely disable ability to save state to those images inside QEMU. Signed-off-by: Denis V. Lunev <den@openvz.org> CC: Kevin Wolf <kwolf@redhat.com> CC: Paolo Bonzini <pbonzini@redhat.com> CC: Laszlo Ersek <lersek@redhat.com> CC: qemu-block@nongnu.org --- block/snapshot.c | 7 +++++++ 1 file changed, 7 insertions(+)