On Apr 3, 2010, at 5:47 PM, Ragnar Sundblad wrote: > On 2 apr 2010, at 22.47, Neil Perrin wrote: > >>> Suppose there is an application which sometimes does sync writes, and >>> sometimes async writes. In fact, to make it easier, suppose two processes >>> open two files, one of which always writes asynchronously, and one of which >>> always writes synchronously. Suppose the ZIL is disabled. Is it possible >>> for writes to be committed to disk out-of-order? Meaning, can a large block >>> async write be put into a TXG and committed to disk before a small sync >>> write to a different file is committed to disk, even though the small sync >>> write was issued by the application before the large async write? Remember, >>> the point is: ZIL is disabled. Question is whether the async could >>> possibly be committed to disk before the sync. >>> >>> >> >> Threads can be pre-empted in the OS at any time. So even though thread A >> issued >> W1 before thread B issued W2, the order is not guaranteed to arrive at ZFS >> as W1, W2. >> Multi-threaded applications have to handle this. >> >> If this was a single thread issuing W1 then W2 then yes the order is >> guaranteed >> regardless of whether W1 or W2 are synchronous or asynchronous. >> Of course if the system crashes then the async operations might not be there. > > Could you please clarify this last paragraph a little: > Do you mean that this is in the case that you have ZIL enabled > and the txg for W1 and W2 hasn't been commited, so that upon reboot > the ZIL is replayed, and therefore only the sync writes are > eventually there?
yes. The ZIL needs to be replayed on import after an unclean shutdown. > If, lets say, W1 is an async small write, W2 is a sync small write, > W1 arrives to zfs before W2, and W2 arrives before the txg is > commited, will both writes always be in the txg on disk? yes > If so, it would mean that zfs itself never buffer up async writes to > larger blurbs to write at a later txg, correct? correct > I take it that ZIL enabled or not does not make any difference here > (we pretend the system did _not_ crash), correct? For import following a clean shutdown, there are no transactions in the ZIL to apply. For async-only workloads, there are no transactions in the ZIL to apply. Do not assume that power outages are the only cause of unclean shutdowns. -- richard ZFS storage and performance consulting at http://www.RichardElling.com ZFS training on deduplication, NexentaStor, and NAS performance Las Vegas, April 29-30, 2010 http://nexenta-vegas.eventbrite.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
