On Thu, 12 May 2016, Igor Fedotov wrote: > On 11.05.2016 23:54, Sage Weil wrote: > > On Wed, 11 May 2016, Sage Weil wrote: > > > On Wed, 11 May 2016, Igor Fedotov wrote: > > > > > > I like that way better! We can just add a force_sync argument to > > > _do_write. That also lets us trivially disable wal (by forcing sync w/ > > > a config option or whatever). > > > > > > The downside is that any logically conflicting request (an overlapping > > > write or truncate or zero) needs to drain the wal events, whereas with a > > > lower-level wal description there might be cases where we can ignore the > > > wal operation. I suspect the trivial solution of o->flush() on > > > write/truncate/zero will be pretty visible in benchmarks. Tracking > > > in-flight wal ops with an interval_set would probably work well enough. > > Hmm, I'm not sure this will pan out. The main problem is that if we call > > back into the write code (with a sync flag), we will have to do write > > IO, and this wreaks havoc on our otherwise (mostly) orderly state machine. > > I think it can be done if we build in a similar guard like _txc_finish_io > > so that we wait for the wal events to also complete IO in order before > > committing them. I think. > > > > But the other problem is the checksum thing that came up in another > > thread, where the read-side of a read/modify/write might fail teh checksum > > because the wal write hit disk but the kv portion didn't commit. I see a > > few options: > > > > 1) If there are checksums and we're doing a sub-block overwrite, we > > have to write/cow it elsewhere. This probably means min_alloc_size cow > > operations for small writes. In which case we needn't bother doing a wal > > even in the first place--the whole point is to enable an overwrite. > > > > 2) We do loose checksum validation that will accept either the old > > checksum or the expected new checksum for the read stage. This handles > > these two crash cases: > Probably I missed something but It seems to me that we don't have any > 'expected new checksum' for the whole new block after the crash. > What we can have are old block checksum in KV and checksum for overwritten > portion of the block in WAL. To have full new checksum one has to do the read > and store new checksum to KV afterwards. > Or you mean write+KV update under 'do wal io'? Yeah, you're right, I'm speaking nonsense! Bottom line, we can't do partial checksum-block r/m/w overwrites (unless the read part of the r/m/w happens beforehand, turning it into a full block overwrite). sage -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
