[ping!] on 03/03/2014 11:17 Andriy Gapon said the following: > > 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 > a vdev 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 on-disk > size of the L2ARC buffers could be quite larger than their logical 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 > > I think that the following FreeBSD changes might also be of interest to > OpenZFS > community: > https://github.com/freebsd/freebsd/commit/bd47afb2893#diff-23 > https://github.com/freebsd/freebsd/commit/1c55b38ae >
-- Andriy Gapon
_______________________________________________ developer mailing list [email protected] http://lists.open-zfs.org/mailman/listinfo/developer
