On 01/08/2010 02:42 PM, Lutz Schumann wrote: > See the reads on the pool with the low I/O ? I suspect reading the > DDT causes the writes to slow down. > > See this bug > http://bugs.opensolaris.org/bugdatabase/view_bug.do?bug_id=6913566. > It seems to give some backgrounds. > > Can you test setting the "primarycache=metadata" on the volume you > test ? This would be my initial test. My suggestion would be that it > may improve the situation because your ARC can be better utilized for > DDT (this does not make much sence for production without a SSD > cache, because you practially disable all caches for reading without > a L2ARC (aka SSD)!) > > As I read the bug report above - it seems the if the DDT > (deduplication table) does not fit into memory or dropped from there > the DDT has to be read from disk causing massive random I/O.
The symptoms described in that bug report do match up with mine. I have also experienced long hang times (>1hr) destroying a dataset while the disk just thrashes. I tried setting "primarycache=metadata", but that did not help. I pulled the DDT statistics for my pool, but don't know how to determine its physical size-on-disk from that. If deduplication ends up requiring a separate sort-of log device, that will be a real shame. > # zdb -DD nest > DDT-sha256-zap-duplicate: 780321 entries, size 338 on disk, 174 in core > DDT-sha256-zap-unique: 6188123 entries, size 335 on disk, 164 in core > > DDT histogram (aggregated over all DDTs): > > bucket allocated referenced > ______ ______________________________ ______________________________ > refcnt blocks LSIZE PSIZE DSIZE blocks LSIZE PSIZE DSIZE > ------ ------ ----- ----- ----- ------ ----- ----- ----- > 1 5.90M 752G 729G 729G 5.90M 752G 729G 729G > 2 756K 94.0G 93.7G 93.6G 1.48M 188G 187G 187G > 4 5.36K 152M 80.3M 81.5M 22.4K 618M 325M 330M > 8 258 4.05M 1.93M 2.00M 2.43K 36.7M 16.3M 16.9M > 16 30 434K 42K 50.9K 597 10.2M 824K 1003K > 32 5 255K 65.5K 66.6K 204 10.5M 3.26M 3.30M > 64 20 2.02M 906K 910K 1.41K 141M 62.0M 62.2M > 128 4 2K 2K 2.99K 723 362K 362K 541K > 256 1 512 512 766 277 138K 138K 207K > 512 2 1K 1K 1.50K 1.62K 830K 830K 1.21M > Total 6.65M 846G 823G 823G 7.41M 941G 917G 917G > > dedup = 1.11, compress = 1.03, copies = 1.00, dedup * compress / copies = 1.14 _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
