On 07/08/2016 05:05 AM, Kevin Wolf wrote: > Am 21.06.2016 um 01:39 hat Eric Blake geschrieben: >> The raw format layer supports all flags via passthrough - but >> it only makes sense to pass through flags that the lower layer >> actually supports. >> >> Thanks to the previous patch, the raw format layer now attempts >> to fragment writes at the max_transfer limit it inherits from >> the NBD protocol layer, recently set to 32m. An attempt to do >> 'w -f 0 40m' to an NBD server that lacks FUA thus changed from >> flushing once (after NBD fragmented a single 40m write itself) >> to instead flushing twice (the format layer sees BDRV_REQ_FUA >> in supported_write_flags, so it sends the flag on to both >> fragments, and then the block layer emulates FUA by flushing >> for both the 32m and 8m fragments at the protocol layer). >> This patch fixes the performance regression (now that the >> format layer no longer advertises a flag not present at the >> protocol layer, the flush to emulate FUA is deferred to the >> last fragment). >> >> Note that 'w -f -z 0 40m' does not currently exhibit the same >> problem, because there, the fragmentation does not occur until >> at the NBD layer (the raw layer has .bdrv_co_pwrite_zeroes, and >> the NBD layer doesn't advertise max_pwrite_zeroes to constrain >> things at the raw layer) - but that problem is latent and would >> have the same problem with too many flushes without this patch >> once the NBD layer implements support for using the new >> NBD_CMD_WRITE_ZEROES and sets max_pwrite_zeroes to the same 32m >> limit as recommended by the NBD protocol. >> >> Signed-off-by: Eric Blake <[email protected]> > > Should this be moved before patch 2 so that we never get a regression in > the first place?
Can do, although it will require some word-smithing to the commit message. -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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