From a5041e1d3c2b1ab5edab3e26763ea6f6b7a5348b Mon Sep 17 00:00:00 2001 From: Andriy Gapon Date: Fri, 12 Jun 2015 21:20:29 +0200 Subject: [PATCH] account for ashift when gathering buffers to be written to l2arc device The problem is that since OpenSolaris commit illumos/illumos-gate@e14bb3258d05c1b1077e2db7cf77088924e56919 l2ad_hand is kept aligned based on ashift (which is derived from the cache device's logical and physical block sizes). So, the hand could be advanced by more than the sum of b_asize-s of the written L2ARC buffers. This is because b_asize is a misnomer at the moment as it does not always represent the allocated size: if a buffer is compressed, then the compressed size is properly rounded, 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 may 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). PR: 198242 PR: 195746 (possibly related) Porting notes: Rather difficult to track changes related to: Illumos 5369 - arc flags should be an enum and Illumos 5408 - managing ZFS cache devices requires lots of RAM hdr->b_l2hdr = l2hdr; changed to hdr->b_flags |= ARC_FLAG_HAS_L2HDR; list_insert_head(dev->l2ad_buflist, hdr); changed to list_insert_head(&dev->l2ad_buflist, hdr); References: https://reviews.freebsd.org/D2764 https://github.com/zfsonlinux/zfs/issues/3400 https://github.com/zfsonlinux/zfs/pull/3433 https://github.com/zfsonlinux/zfs/pull/3451 Ported by: kernelOfTruth --- module/zfs/arc.c | 47 +++++++++++++++++++++++++++++++++++------------ 1 file changed, 35 insertions(+), 12 deletions(-) diff --git a/module/zfs/arc.c b/module/zfs/arc.c index 805e7b59f5f7..dd285de3c83d 100644 --- a/module/zfs/arc.c +++ b/module/zfs/arc.c @@ -5723,8 +5723,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, boolean_t *headroom_boost) { arc_buf_hdr_t *hdr, *hdr_prev, *head; - uint64_t write_asize, write_psize, write_sz, headroom, - buf_compress_minsz; + uint64_t write_asize, write_sz, headroom, buf_compress_minsz; void *buf_data; boolean_t full; l2arc_write_callback_t *cb; @@ -5739,7 +5738,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, *headroom_boost = B_FALSE; pio = NULL; - write_sz = write_asize = write_psize = 0; + write_sz = write_asize = 0; full = B_FALSE; head = kmem_cache_alloc(hdr_l2only_cache, KM_PUSHPAGE); head->b_flags |= ARC_FLAG_L2_WRITE_HEAD; @@ -5776,6 +5775,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, for (; hdr; hdr = hdr_prev) { kmutex_t *hash_lock; uint64_t buf_sz; + uint64_t buf_a_sz; if (arc_warm == B_FALSE) hdr_prev = multilist_sublist_next(mls, hdr); @@ -5804,7 +5804,15 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, continue; } - if ((write_sz + hdr->b_size) > target_sz) { + /* + * Assume that the buffer is not going to be compressed + * and could take more space on disk because of a larger + * disk block size. + */ + buf_sz = hdr->b_size; + buf_a_sz = vdev_psize_to_asize(dev->l2ad_vdev, buf_sz); + + if ((write_asize + buf_a_sz) > target_sz) { full = B_TRUE; mutex_exit(hash_lock); break; @@ -5847,7 +5855,6 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, hdr->b_l2hdr.b_hits = 0; hdr->b_l1hdr.b_tmp_cdata = hdr->b_l1hdr.b_buf->b_data; - buf_sz = hdr->b_size; hdr->b_flags |= ARC_FLAG_HAS_L2HDR; mutex_enter(&dev->l2ad_mtx); @@ -5864,6 +5871,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, mutex_exit(hash_lock); write_sz += buf_sz; + write_asize += buf_a_sz; } multilist_sublist_unlock(mls); @@ -5882,6 +5890,20 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, mutex_enter(&dev->l2ad_mtx); + /* + * Note that elsewhere in this file arcstat_l2_asize + * and the used space on l2ad_vdev are updated using b_asize, + * which is not necessarily rounded up to the device block size. + * Too keep accounting consistent we do the same here as well: + * stats_size accumulates the sum of b_asize of the written buffers, + * while write_asize accumulates the sum of b_asize rounded up + * to the device block size. + * The latter sum is used only to validate the corectness of the code. + */ + uint64_t stats_size; + stats_size = 0; + write_asize = 0; + /* * Now start writing the buffers. We're starting at the write head * and work backwards, retracing the course of the buffer selector @@ -5928,7 +5950,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, /* Compression may have squashed the buffer to zero length. */ if (buf_sz != 0) { - uint64_t buf_p_sz; + uint64_t buf_a_sz; wzio = zio_write_phys(pio, dev->l2ad_vdev, dev->l2ad_hand, buf_sz, buf_data, ZIO_CHECKSUM_OFF, @@ -5939,13 +5961,14 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, zio_t *, wzio); (void) zio_nowait(wzio); - write_asize += buf_sz; + stats_size += buf_sz; + /* * 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; + buf_a_sz = vdev_psize_to_asize(dev->l2ad_vdev, buf_sz); + write_asize += buf_a_sz; + dev->l2ad_hand += buf_a_sz; } } @@ -5955,8 +5978,8 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz, ARCSTAT_BUMP(arcstat_l2_writes_sent); ARCSTAT_INCR(arcstat_l2_write_bytes, write_asize); ARCSTAT_INCR(arcstat_l2_size, write_sz); - ARCSTAT_INCR(arcstat_l2_asize, write_asize); - vdev_space_update(dev->l2ad_vdev, write_asize, 0, 0); + ARCSTAT_INCR(arcstat_l2_asize, stats_size); + vdev_space_update(dev->l2ad_vdev, stats_size, 0, 0); /* * Bump device hand to the device start if it is approaching the end.