On Mon, Aug 24, 2015 at 08:10:52AM +0000, Andriy Gapon wrote: > Author: avg > Date: Mon Aug 24 08:10:52 2015 > New Revision: 287099 > URL: https://svnweb.freebsd.org/changeset/base/287099 > > Log: > account for ashift when gathering buffers to be written to l2arc device > > The change that introduced the L2ARC compression support also introduced > a bug where the on-disk size of the selected buffers could end up larger > than the target size if the ashift is greater than 9. This was because > the buffer selection could did not take into account the fact that > on-disk size could be larger than the in-memory buffer size due to > the alignment requirements. > > At the moment b_asize is a misnomer as it does not always represent the > allocated size: if a buffer is compressed, then the compressed size is > properly rounded (on FreeBSD), but if the compression fails or it is not > applied, then the original size is kept and it could be smaller than what > ashift requires. > > For the same reasons arcstat_l2_asize and the reported used space > on the cache device could be smaller than the actual allocated size > if ashift > 9. That problem is not fixed by this change. > > This change only ensures that l2ad_hand is not advanced by more > than target_sz. Otherwise we would overwrite active (unevicted) > L2ARC buffers. That problem is manifested as growing l2_cksum_bad > and l2_io_error counters. > > This change also changes 'p' prefix to 'a' prefix in a few places > where variables represent allocated rather than physical size. > > The resolved problem could also result in the reported allocated size > being greater than the cache device's capacity, because of the > overwritten buffers (more than one buffer claiming the same disk > space). > > This change is already in ZFS-on-Linux: > zfsonlinux/zfs@ef56b0780c80ebb0b1e637b8b8c79530a8ab3201 > > PR: 198242 > PR: 195746 (possibly related) > Reviewed by: mahrens (https://reviews.csiden.org/r/229/) > Tested by: gkon...@aicom.gr (most recently) > MFC after: 15 days
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