| Message ID | 20180704112302.471456-3-vsementsov@virtuozzo.com |
|---|---|
| State | New |
| Headers | show |
| Series | nbd base:allocation several extents | expand |
On 07/04/2018 06:23 AM, Vladimir Sementsov-Ogievskiy wrote: > This is necessary for efficient block-status export, for clients which > support it. qemu as client doesn't currently process additional information when querying block status. Should it? Are there other clients which DO make use of the additional information? This is a feature, so it is indeed 3.1 material. > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> > --- > nbd/server.c | 77 +++++++++++++++++++++++++++++++++++++++++++++--------------- > 1 file changed, 58 insertions(+), 19 deletions(-) > > diff --git a/nbd/server.c b/nbd/server.c > index ea5fe0eb33..a6d013aec4 100644 > --- a/nbd/server.c > +++ b/nbd/server.c > @@ -1844,11 +1844,22 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client, > return ret; > } > > -static int blockstatus_to_extent_be(BlockDriverState *bs, uint64_t offset, > - uint64_t bytes, NBDExtent *extent) > +/* > + * Populate @extents from block status. Store in @bytes the byte length encoded > + * (which may be smaller but (unlike bitmap_to_extents) _never_ larger than the > + * original), and store in @nb_extents the number of extents used. I don't think the comparison to bitmap_to_extents in a nested parenthetical buys us anything; the shorter: (which may be smaller than the original) would do just fine for the function contract. > + * > + * Returns zero on success and -errno on bdrv_block_status_above failure. Is it worth returning a positive value for one less pointer parameter used for output? > + */ > +static int blockstatus_to_extents(BlockDriverState *bs, uint64_t offset, > + uint64_t *bytes, NBDExtent *extents, > + unsigned int *nb_extents) > { > - uint64_t remaining_bytes = bytes; > + uint64_t remaining_bytes = *bytes; > + NBDExtent *extent = extents, *extents_end = extents + *nb_extents; So both bytes and nb_extents are in-out. > @@ -1910,21 +1940,29 @@ static int nbd_co_send_extents(NBDClient *client, uint64_t handle, > /* Get block status from the exported device and send it to the client */ > static int nbd_co_send_block_status(NBDClient *client, uint64_t handle, > BlockDriverState *bs, uint64_t offset, > - uint32_t length, bool last, > - uint32_t context_id, Error **errp) > + uint32_t length, bool dont_fragment, > + bool last, uint32_t context_id, > + Error **errp) > { > int ret; > - NBDExtent extent; > + unsigned int nb_extents = dont_fragment ? 1 : NBD_MAX_BITMAP_EXTENTS; > + NBDExtent *extents = g_new(NBDExtent, nb_extents); > + uint64_t final_length = length; > > - ret = blockstatus_to_extent_be(bs, offset, length, &extent); > + ret = blockstatus_to_extents(bs, offset, &final_length, extents, > + &nb_extents); > if (ret < 0) { > + g_free(extents); > return nbd_co_send_structured_error( > client, handle, -ret, "can't get block status", errp); > } > > - return nbd_co_send_extents(client, handle, &extent, 1, > - be32_to_cpu(extent.length), last, > - context_id, errp); > + ret = nbd_co_send_extents(client, handle, extents, nb_extents, > + final_length, last, context_id, errp); > + > + g_free(extents); > + At any rate, this conversion looks sane. > + return ret; > } > > /* > @@ -2228,10 +2266,11 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, > (client->export_meta.base_allocation || > client->export_meta.bitmap)) > { > + bool dont_fragment = request->flags & NBD_CMD_FLAG_REQ_ONE; > if (client->export_meta.base_allocation) { Blank lines between declarations and statements can aid in legibility; I can add when queuing. Reviewed-by: Eric Blake <eblake@redhat.com>
05.07.2018 18:59, Eric Blake wrote: > On 07/04/2018 06:23 AM, Vladimir Sementsov-Ogievskiy wrote: >> This is necessary for efficient block-status export, for clients which >> support it. > > qemu as client doesn't currently process additional information when > querying block status. Should it? > > Are there other clients which DO make use of the additional information? Actually, we have one in development. block status export is used for external full backups, like bitmap export - for external incremental backups. > > This is a feature, so it is indeed 3.1 material. > >> >> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> >> --- >> nbd/server.c | 77 >> +++++++++++++++++++++++++++++++++++++++++++++--------------- >> 1 file changed, 58 insertions(+), 19 deletions(-) >> >> diff --git a/nbd/server.c b/nbd/server.c >> index ea5fe0eb33..a6d013aec4 100644 >> --- a/nbd/server.c >> +++ b/nbd/server.c >> @@ -1844,11 +1844,22 @@ static int coroutine_fn >> nbd_co_send_sparse_read(NBDClient *client, >> return ret; >> } >> -static int blockstatus_to_extent_be(BlockDriverState *bs, uint64_t >> offset, >> - uint64_t bytes, NBDExtent *extent) >> +/* >> + * Populate @extents from block status. Store in @bytes the byte >> length encoded >> + * (which may be smaller but (unlike bitmap_to_extents) _never_ >> larger than the >> + * original), and store in @nb_extents the number of extents used. > > I don't think the comparison to bitmap_to_extents in a nested > parenthetical buys us anything; the shorter: > > (which may be smaller than the original) > > would do just fine for the function contract. > >> + * >> + * Returns zero on success and -errno on bdrv_block_status_above >> failure. > > Is it worth returning a positive value for one less pointer parameter > used for output? I think, that if we have one such parameter anyway, let's use same path for very similar thing, for consistency. > >> + */ >> +static int blockstatus_to_extents(BlockDriverState *bs, uint64_t >> offset, >> + uint64_t *bytes, NBDExtent *extents, >> + unsigned int *nb_extents) >> { >> - uint64_t remaining_bytes = bytes; >> + uint64_t remaining_bytes = *bytes; >> + NBDExtent *extent = extents, *extents_end = extents + *nb_extents; > > So both bytes and nb_extents are in-out. > > >> @@ -1910,21 +1940,29 @@ static int nbd_co_send_extents(NBDClient >> *client, uint64_t handle, >> /* Get block status from the exported device and send it to the >> client */ >> static int nbd_co_send_block_status(NBDClient *client, uint64_t >> handle, >> BlockDriverState *bs, uint64_t >> offset, >> - uint32_t length, bool last, >> - uint32_t context_id, Error **errp) >> + uint32_t length, bool >> dont_fragment, >> + bool last, uint32_t context_id, >> + Error **errp) >> { >> int ret; >> - NBDExtent extent; >> + unsigned int nb_extents = dont_fragment ? 1 : >> NBD_MAX_BITMAP_EXTENTS; >> + NBDExtent *extents = g_new(NBDExtent, nb_extents); >> + uint64_t final_length = length; >> - ret = blockstatus_to_extent_be(bs, offset, length, &extent); >> + ret = blockstatus_to_extents(bs, offset, &final_length, extents, >> + &nb_extents); >> if (ret < 0) { >> + g_free(extents); >> return nbd_co_send_structured_error( >> client, handle, -ret, "can't get block status", errp); >> } >> - return nbd_co_send_extents(client, handle, &extent, 1, >> - be32_to_cpu(extent.length), last, >> - context_id, errp); >> + ret = nbd_co_send_extents(client, handle, extents, nb_extents, >> + final_length, last, context_id, errp); >> + >> + g_free(extents); >> + > > At any rate, this conversion looks sane. > >> + return ret; >> } >> /* >> @@ -2228,10 +2266,11 @@ static coroutine_fn int >> nbd_handle_request(NBDClient *client, >> (client->export_meta.base_allocation || >> client->export_meta.bitmap)) >> { >> + bool dont_fragment = request->flags & NBD_CMD_FLAG_REQ_ONE; >> if (client->export_meta.base_allocation) { > > Blank lines between declarations and statements can aid in legibility; > I can add when queuing. > > Reviewed-by: Eric Blake <eblake@redhat.com> > Ok, thank you!
04.07.2018 14:23, Vladimir Sementsov-Ogievskiy wrote: > This is necessary for efficient block-status export, for clients which > support it. > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> > --- > nbd/server.c | 77 +++++++++++++++++++++++++++++++++++++++++++++--------------- > 1 file changed, 58 insertions(+), 19 deletions(-) > > diff --git a/nbd/server.c b/nbd/server.c > index ea5fe0eb33..a6d013aec4 100644 > --- a/nbd/server.c > +++ b/nbd/server.c > @@ -1844,11 +1844,22 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client, > return ret; > } > > -static int blockstatus_to_extent_be(BlockDriverState *bs, uint64_t offset, > - uint64_t bytes, NBDExtent *extent) > +/* > + * Populate @extents from block status. Store in @bytes the byte length encoded > + * (which may be smaller but (unlike bitmap_to_extents) _never_ larger than the > + * original), and store in @nb_extents the number of extents used. > + * > + * Returns zero on success and -errno on bdrv_block_status_above failure. > + */ > +static int blockstatus_to_extents(BlockDriverState *bs, uint64_t offset, > + uint64_t *bytes, NBDExtent *extents, > + unsigned int *nb_extents) > { > - uint64_t remaining_bytes = bytes; > + uint64_t remaining_bytes = *bytes; > + NBDExtent *extent = extents, *extents_end = extents + *nb_extents; > + bool first_extent = true; > > + assert(*nb_extents); > while (remaining_bytes) { > uint32_t flags; > int64_t num; > @@ -1860,21 +1871,40 @@ static int blockstatus_to_extent_be(BlockDriverState *bs, uint64_t offset, after bdrv_block_status_above, is there a guarantee that num > 0? Should we add an assertion? > > flags = (ret & BDRV_BLOCK_ALLOCATED ? 0 : NBD_STATE_HOLE) | > (ret & BDRV_BLOCK_ZERO ? NBD_STATE_ZERO : 0); > + offset += num; > + remaining_bytes -= num; > > - if (remaining_bytes == bytes) { > + if (first_extent) { > extent->flags = flags; > + extent->length = num; > + first_extent = false; > + continue; > } > > - if (flags != extent->flags) { > - break; > + if (flags == extent->flags) { > + /* extend current extent */ > + extent->length += num; > + } else { > + if (extent + 1 == extents_end) { > + break; > + } > + > + /* start new extent */ > + extent++; > + extent->flags = flags; > + extent->length = num; > } > + } > > - offset += num; > - remaining_bytes -= num; > + extents_end = extent + 1; > + > + for (extent = extents; extent < extents_end; extent++) { > + cpu_to_be32s(&extent->flags); > + cpu_to_be32s(&extent->length); > } > > - cpu_to_be32s(&extent->flags); > - extent->length = cpu_to_be32(bytes - remaining_bytes); > + *bytes -= remaining_bytes; > + *nb_extents = extents_end - extents; > > return 0; > } > @@ -1910,21 +1940,29 @@ static int nbd_co_send_extents(NBDClient *client, uint64_t handle, > /* Get block status from the exported device and send it to the client */ > static int nbd_co_send_block_status(NBDClient *client, uint64_t handle, > BlockDriverState *bs, uint64_t offset, > - uint32_t length, bool last, > - uint32_t context_id, Error **errp) > + uint32_t length, bool dont_fragment, > + bool last, uint32_t context_id, > + Error **errp) > { > int ret; > - NBDExtent extent; > + unsigned int nb_extents = dont_fragment ? 1 : NBD_MAX_BITMAP_EXTENTS; > + NBDExtent *extents = g_new(NBDExtent, nb_extents); > + uint64_t final_length = length; > > - ret = blockstatus_to_extent_be(bs, offset, length, &extent); > + ret = blockstatus_to_extents(bs, offset, &final_length, extents, > + &nb_extents); > if (ret < 0) { > + g_free(extents); > return nbd_co_send_structured_error( > client, handle, -ret, "can't get block status", errp); > } > > - return nbd_co_send_extents(client, handle, &extent, 1, > - be32_to_cpu(extent.length), last, > - context_id, errp); > + ret = nbd_co_send_extents(client, handle, extents, nb_extents, > + final_length, last, context_id, errp); > + > + g_free(extents); > + > + return ret; > } > > /* > @@ -2228,10 +2266,11 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, > (client->export_meta.base_allocation || > client->export_meta.bitmap)) > { > + bool dont_fragment = request->flags & NBD_CMD_FLAG_REQ_ONE; > if (client->export_meta.base_allocation) { > ret = nbd_co_send_block_status(client, request->handle, > blk_bs(exp->blk), request->from, > - request->len, > + request->len, dont_fragment, > !client->export_meta.bitmap, > NBD_META_ID_BASE_ALLOCATION, > errp); > @@ -2244,7 +2283,7 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, > ret = nbd_co_send_bitmap(client, request->handle, > client->exp->export_bitmap, > request->from, request->len, > - request->flags & NBD_CMD_FLAG_REQ_ONE, > + dont_fragment, > true, NBD_META_ID_DIRTY_BITMAP, errp); > if (ret < 0) { > return ret;
On 07/10/2018 09:33 AM, Vladimir Sementsov-Ogievskiy wrote: > 04.07.2018 14:23, Vladimir Sementsov-Ogievskiy wrote: >> This is necessary for efficient block-status export, for clients which >> support it. >> >> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> >> --- >> +static int blockstatus_to_extents(BlockDriverState *bs, uint64_t offset, >> + uint64_t *bytes, NBDExtent *extents, >> + unsigned int *nb_extents) >> { >> - uint64_t remaining_bytes = bytes; >> + uint64_t remaining_bytes = *bytes; >> + NBDExtent *extent = extents, *extents_end = extents + *nb_extents; >> + bool first_extent = true; >> + assert(*nb_extents); >> while (remaining_bytes) { >> uint32_t flags; >> int64_t num; >> @@ -1860,21 +1871,40 @@ static int >> blockstatus_to_extent_be(BlockDriverState *bs, uint64_t offset, > > after bdrv_block_status_above, is there a guarantee that num > 0? Should > we add an assertion? bdrv_block_status_above() asserts that drivers set *pnum to non-zero on success (that is, a driver's .bdrv_co_block_status must make progress or fail); however, a caller can still get *pnum == 0 on the corner case of requesting status at or beyond the end-of-file boundary (where bdrv_block_status_above() does not call into a driver's .bdrv_co_block_status callback). Since the NBD code already validated that the client's code does not exceed EOF, and the block layer guaranteed that an in-range request made progress on success, you should be fine with an assertion that num > 0.
05.07.2018 19:18, Vladimir Sementsov-Ogievskiy wrote: > 05.07.2018 18:59, Eric Blake wrote: >> On 07/04/2018 06:23 AM, Vladimir Sementsov-Ogievskiy wrote: >>> This is necessary for efficient block-status export, for clients which >>> support it. >> >> qemu as client doesn't currently process additional information when >> querying block status. Should it? >> >> Are there other clients which DO make use of the additional information? > > Actually, we have one in development. block status export is used for > external full backups, like bitmap export - for external incremental > backups. > >> >> This is a feature, so it is indeed 3.1 material. >> >>> >>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> >>> --- >>> nbd/server.c | 77 >>> +++++++++++++++++++++++++++++++++++++++++++++--------------- >>> 1 file changed, 58 insertions(+), 19 deletions(-) >>> >>> diff --git a/nbd/server.c b/nbd/server.c >>> index ea5fe0eb33..a6d013aec4 100644 >>> --- a/nbd/server.c >>> +++ b/nbd/server.c >>> @@ -1844,11 +1844,22 @@ static int coroutine_fn >>> nbd_co_send_sparse_read(NBDClient *client, >>> return ret; >>> } >>> -static int blockstatus_to_extent_be(BlockDriverState *bs, >>> uint64_t offset, >>> - uint64_t bytes, NBDExtent *extent) >>> +/* >>> + * Populate @extents from block status. Store in @bytes the byte >>> length encoded >>> + * (which may be smaller but (unlike bitmap_to_extents) _never_ >>> larger than the >>> + * original), and store in @nb_extents the number of extents used. >> >> I don't think the comparison to bitmap_to_extents in a nested >> parenthetical buys us anything; the shorter: >> >> (which may be smaller than the original) >> >> would do just fine for the function contract. >> >>> + * >>> + * Returns zero on success and -errno on bdrv_block_status_above >>> failure. >> >> Is it worth returning a positive value for one less pointer parameter >> used for output? > > I think, that if we have one such parameter anyway, let's use same > path for very similar thing, for consistency. > >> >>> + */ >>> +static int blockstatus_to_extents(BlockDriverState *bs, uint64_t >>> offset, >>> + uint64_t *bytes, NBDExtent *extents, >>> + unsigned int *nb_extents) >>> { >>> - uint64_t remaining_bytes = bytes; >>> + uint64_t remaining_bytes = *bytes; >>> + NBDExtent *extent = extents, *extents_end = extents + *nb_extents; >> >> So both bytes and nb_extents are in-out. >> >> >>> @@ -1910,21 +1940,29 @@ static int nbd_co_send_extents(NBDClient >>> *client, uint64_t handle, >>> /* Get block status from the exported device and send it to the >>> client */ >>> static int nbd_co_send_block_status(NBDClient *client, uint64_t >>> handle, >>> BlockDriverState *bs, uint64_t >>> offset, >>> - uint32_t length, bool last, >>> - uint32_t context_id, Error **errp) >>> + uint32_t length, bool >>> dont_fragment, >>> + bool last, uint32_t context_id, >>> + Error **errp) >>> { >>> int ret; >>> - NBDExtent extent; >>> + unsigned int nb_extents = dont_fragment ? 1 : >>> NBD_MAX_BITMAP_EXTENTS; >>> + NBDExtent *extents = g_new(NBDExtent, nb_extents); >>> + uint64_t final_length = length; >>> - ret = blockstatus_to_extent_be(bs, offset, length, &extent); >>> + ret = blockstatus_to_extents(bs, offset, &final_length, extents, >>> + &nb_extents); >>> if (ret < 0) { >>> + g_free(extents); >>> return nbd_co_send_structured_error( >>> client, handle, -ret, "can't get block status", >>> errp); >>> } >>> - return nbd_co_send_extents(client, handle, &extent, 1, >>> - be32_to_cpu(extent.length), last, >>> - context_id, errp); >>> + ret = nbd_co_send_extents(client, handle, extents, nb_extents, >>> + final_length, last, context_id, errp); >>> + >>> + g_free(extents); >>> + >> >> At any rate, this conversion looks sane. >> >>> + return ret; >>> } >>> /* >>> @@ -2228,10 +2266,11 @@ static coroutine_fn int >>> nbd_handle_request(NBDClient *client, >>> (client->export_meta.base_allocation || >>> client->export_meta.bitmap)) >>> { >>> + bool dont_fragment = request->flags & >>> NBD_CMD_FLAG_REQ_ONE; >>> if (client->export_meta.base_allocation) { >> >> Blank lines between declarations and statements can aid in >> legibility; I can add when queuing. >> >> Reviewed-by: Eric Blake <eblake@redhat.com> >> > > Ok, thank you! > hmm, looks like lost patch :(
On 9/17/18 11:15 AM, Vladimir Sementsov-Ogievskiy wrote: > 05.07.2018 19:18, Vladimir Sementsov-Ogievskiy wrote: >> 05.07.2018 18:59, Eric Blake wrote: >>> On 07/04/2018 06:23 AM, Vladimir Sementsov-Ogievskiy wrote: >>>> This is necessary for efficient block-status export, for clients which >>>> support it. >>> >>> >>> This is a feature, so it is indeed 3.1 material. >>> Reviewed-by: Eric Blake <eblake@redhat.com> >>> >> >> Ok, thank you! >> > > hmm, looks like lost patch :( Not lost, just extremely slow getting queued. Now on my NBD queue.
diff --git a/nbd/server.c b/nbd/server.c index ea5fe0eb33..a6d013aec4 100644 --- a/nbd/server.c +++ b/nbd/server.c @@ -1844,11 +1844,22 @@ static int coroutine_fn nbd_co_send_sparse_read(NBDClient *client, return ret; } -static int blockstatus_to_extent_be(BlockDriverState *bs, uint64_t offset, - uint64_t bytes, NBDExtent *extent) +/* + * Populate @extents from block status. Store in @bytes the byte length encoded + * (which may be smaller but (unlike bitmap_to_extents) _never_ larger than the + * original), and store in @nb_extents the number of extents used. + * + * Returns zero on success and -errno on bdrv_block_status_above failure. + */ +static int blockstatus_to_extents(BlockDriverState *bs, uint64_t offset, + uint64_t *bytes, NBDExtent *extents, + unsigned int *nb_extents) { - uint64_t remaining_bytes = bytes; + uint64_t remaining_bytes = *bytes; + NBDExtent *extent = extents, *extents_end = extents + *nb_extents; + bool first_extent = true; + assert(*nb_extents); while (remaining_bytes) { uint32_t flags; int64_t num; @@ -1860,21 +1871,40 @@ static int blockstatus_to_extent_be(BlockDriverState *bs, uint64_t offset, flags = (ret & BDRV_BLOCK_ALLOCATED ? 0 : NBD_STATE_HOLE) | (ret & BDRV_BLOCK_ZERO ? NBD_STATE_ZERO : 0); + offset += num; + remaining_bytes -= num; - if (remaining_bytes == bytes) { + if (first_extent) { extent->flags = flags; + extent->length = num; + first_extent = false; + continue; } - if (flags != extent->flags) { - break; + if (flags == extent->flags) { + /* extend current extent */ + extent->length += num; + } else { + if (extent + 1 == extents_end) { + break; + } + + /* start new extent */ + extent++; + extent->flags = flags; + extent->length = num; } + } - offset += num; - remaining_bytes -= num; + extents_end = extent + 1; + + for (extent = extents; extent < extents_end; extent++) { + cpu_to_be32s(&extent->flags); + cpu_to_be32s(&extent->length); } - cpu_to_be32s(&extent->flags); - extent->length = cpu_to_be32(bytes - remaining_bytes); + *bytes -= remaining_bytes; + *nb_extents = extents_end - extents; return 0; } @@ -1910,21 +1940,29 @@ static int nbd_co_send_extents(NBDClient *client, uint64_t handle, /* Get block status from the exported device and send it to the client */ static int nbd_co_send_block_status(NBDClient *client, uint64_t handle, BlockDriverState *bs, uint64_t offset, - uint32_t length, bool last, - uint32_t context_id, Error **errp) + uint32_t length, bool dont_fragment, + bool last, uint32_t context_id, + Error **errp) { int ret; - NBDExtent extent; + unsigned int nb_extents = dont_fragment ? 1 : NBD_MAX_BITMAP_EXTENTS; + NBDExtent *extents = g_new(NBDExtent, nb_extents); + uint64_t final_length = length; - ret = blockstatus_to_extent_be(bs, offset, length, &extent); + ret = blockstatus_to_extents(bs, offset, &final_length, extents, + &nb_extents); if (ret < 0) { + g_free(extents); return nbd_co_send_structured_error( client, handle, -ret, "can't get block status", errp); } - return nbd_co_send_extents(client, handle, &extent, 1, - be32_to_cpu(extent.length), last, - context_id, errp); + ret = nbd_co_send_extents(client, handle, extents, nb_extents, + final_length, last, context_id, errp); + + g_free(extents); + + return ret; } /* @@ -2228,10 +2266,11 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, (client->export_meta.base_allocation || client->export_meta.bitmap)) { + bool dont_fragment = request->flags & NBD_CMD_FLAG_REQ_ONE; if (client->export_meta.base_allocation) { ret = nbd_co_send_block_status(client, request->handle, blk_bs(exp->blk), request->from, - request->len, + request->len, dont_fragment, !client->export_meta.bitmap, NBD_META_ID_BASE_ALLOCATION, errp); @@ -2244,7 +2283,7 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, ret = nbd_co_send_bitmap(client, request->handle, client->exp->export_bitmap, request->from, request->len, - request->flags & NBD_CMD_FLAG_REQ_ONE, + dont_fragment, true, NBD_META_ID_DIRTY_BITMAP, errp); if (ret < 0) { return ret;
This is necessary for efficient block-status export, for clients which support it. Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com> --- nbd/server.c | 77 +++++++++++++++++++++++++++++++++++++++++++++--------------- 1 file changed, 58 insertions(+), 19 deletions(-)