on 18/10/2013 17:57 Steven Hartland said the following: > I think we we may well need the following patch to set the minblock > size based on the vdev ashift and not SPA_MINBLOCKSIZE. > > svn diff -x -p sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c > Index: sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c > =================================================================== > --- sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c (revision 256554) > +++ sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c (working copy) > _at__at_ -5147,7 +5147,7 _at__at_ l2arc_compress_buf(l2arc_buf_hdr_t *l2hdr) > len = l2hdr->b_asize; > cdata = zio_data_buf_alloc(len); > csize = zio_compress_data(ZIO_COMPRESS_LZ4, l2hdr->b_tmp_cdata, > - cdata, l2hdr->b_asize, (size_t)SPA_MINBLOCKSIZE); > + cdata, l2hdr->b_asize, (size_t)(1ULL << > l2hdr->b_dev->l2ad_vdev->vdev_ashift)); > > if (csize == 0) { > /* zero block, indicate that there's nothing to write */ This is a rather old thread and change, but I think that I have identified another problem with 4KB cache devices. I noticed that on some of our systems we were getting a clearly abnormal number of l2arc checksum errors accounted in l2_cksum_bad. The hardware appeared to be in good health. Using DTrace I noticed that the data seemed to be overwritten with other data. After more DTrace analysis I observed that sometimes l2arc_write_buffers() would advance l2ad_hand by more than target_sz. This meant that l2arc_write_buffers() would write beyond a region cleared by l2arc_evict() and thus overwrite data belonging to non-evicted buffers. Havoc ensues. The cache devices in question are all SSDs with logical sector size of 4KB. I am not sure about other ZFS platforms, but on FreeBSD this fact is detected and ashift of 12 is used for the cache vdevs. Looking at l2arc_write_buffers() code you can see that it properly accounts for ashift when actually writing buffers and advancing l2ad_hand: /* * Keep the clock hand suitably device-aligned. */ buf_p_sz = vdev_psize_to_asize(dev->l2ad_vdev, buf_sz); write_psize += buf_p_sz; dev->l2ad_hand += buf_p_sz; But the same is not done when selecting buffers to be written and checking that target_sz is not exceeded. So, if ARC contains a lot of buffers smaller than 4K that means that an aligned on-disk size of the L2ARC buffers could be quite larger than their non-aligned size. I propose the following patch which has been tested and seems to fix the problem without introducing any side effects: https://github.com/avg-I/freebsd/compare/review;l2arc-write-target-size.diff https://github.com/avg-I/freebsd/compare/review;l2arc-write-target-size -- Andriy GaponReceived on Mon Mar 03 2014 - 08:17:50 UTC
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