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zfetch: Don't issue new streams when old have not completed

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The current dmu_zfetch code implicitly assumes that I/Os complete
within min_sec_reap seconds. With async dmu and a readonly workload
(and thus no exponential backoff in operations from the "write
throttle") such as L2ARC rebuild it is possible to saturate the drives
with I/O requests. These are then effectively compounded with prefetch
requests.

This change reference counts streams and prevents them from being
recycled after their min_sec_reap timeout if they still have
outstanding I/Os.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #10900
This commit is contained in:
Matthew Macy
2020-09-27 17:08:38 -07:00
committed by GitHub
parent cf2667759f
commit af20b97078
4 changed files with 166 additions and 44 deletions
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include/sys/dmu_zfetch.h
+10 -6
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@@ -40,6 +40,13 @@ extern unsigned long zfetch_array_rd_sz;
struct dnode; /* so we can reference dnode */
typedef struct zfetch {
kmutex_t zf_lock; /* protects zfetch structure */
list_t zf_stream; /* list of zstream_t's */
struct dnode *zf_dnode; /* dnode that owns this zfetch */
int zf_numstreams; /* number of zstream_t's */
} zfetch_t;
typedef struct zstream {
uint64_t zs_blkid; /* expect next access at this blkid */
uint64_t zs_pf_blkid; /* next block to prefetch */
@@ -52,15 +59,12 @@ typedef struct zstream {
kmutex_t zs_lock; /* protects stream */
hrtime_t zs_atime; /* time last prefetch issued */
hrtime_t zs_start_time; /* start of last prefetch */
list_node_t zs_node; /* link for zf_stream */
zfetch_t *zs_fetch; /* parent fetch */
zfs_refcount_t zs_blocks; /* number of pending blocks in the stream */
} zstream_t;
typedef struct zfetch {
kmutex_t zf_lock; /* protects zfetch structure */
list_t zf_stream; /* list of zstream_t's */
struct dnode *zf_dnode; /* dnode that owns this zfetch */
} zfetch_t;
void zfetch_init(void);
void zfetch_fini(void);