2012-01-08 5:45, Richard Elling wrote:
I think you will see a tradeoff on the read side of the mixed read/write workload. Sync writes have higher priority than reads so the order of I/O sent to the disk will appear to be very random and not significantly coalesced. This is the pathological worst case workload for a HDD.
I guess this is what I'm trying to combat when thinking about a dedicated ZIL (SLOG device) in ordedr to reduce pool's fragmentation. It is my understanding (which may be wrong and often is) that without a dedicated SLOG: 1) Sync writes will land on disk randomly into nearest (to disk heads) available blocks, in order to have them committed ASAP; 2) Coalesced writes (at TXG sync) may have intermixed data and metadata blocks, of which metadata may soon expire due to whatever updates, snapshots or deletions involving the blocks this metadata references. If this is true, then after a while there will be many "available" cheese-holes from expired metadata among larger data blocks. 3) Now, this might be further complicated (or relieved) if the metadata blocks are stored in separate groupings from the "bulk" user-data, which I don't know about yet. In that case it would be easier for ZFS to prefetch metadata from disk in one IO (as we discussed in another thread), as well as to effectively reuse the small cheese-holes from freed older metadata blocks. --- If any of the above is true, then it is my "blind expectation" that a dedicated ZIL/SLOG area would decrease fragmentation at least due to sync writes of metadata, and possibly of data, into nearest HDD locations. Again, this is based on my possibly wrong understanding that the blocks committed to a SLOG would be neatly recommitted to the main pool during a TXG close with coalesced writes. I do understand the argument that if the SLOG is dedicated from a certain area on the same HDD, then in fact this would be slowing down the writes by creating more random IO and extra seeks. But as a trade-off I hope for more linear faster reads, including pool import, scrubbing and ZDB walks; and less fragmented free space. Is there any truth to these words? ;) Thanks, //Jim Klimov _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
