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b686631865
Requests that triggers flushing volatile writeback cache to disk (barriers) have significant effect to overall performance. Block layer has sophisticated engine for combining several flush requests into one. But there is no statistics for actual flushes executed by disk. Requests which trigger flushes usually are barriers - zero-size writes. This patch adds two iostat counters into /sys/class/block/$dev/stat and /proc/diskstats - count of completed flush requests and their total time. Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru> Signed-off-by: Jens Axboe <axboe@kernel.dk>
104 lines
4.2 KiB
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104 lines
4.2 KiB
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===============================================
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Block layer statistics in /sys/block/<dev>/stat
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===============================================
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This file documents the contents of the /sys/block/<dev>/stat file.
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The stat file provides several statistics about the state of block
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device <dev>.
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Q.
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Why are there multiple statistics in a single file? Doesn't sysfs
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normally contain a single value per file?
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A.
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By having a single file, the kernel can guarantee that the statistics
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represent a consistent snapshot of the state of the device. If the
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statistics were exported as multiple files containing one statistic
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each, it would be impossible to guarantee that a set of readings
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represent a single point in time.
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The stat file consists of a single line of text containing 11 decimal
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values separated by whitespace. The fields are summarized in the
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following table, and described in more detail below.
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=============== ============= =================================================
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Name units description
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=============== ============= =================================================
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read I/Os requests number of read I/Os processed
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read merges requests number of read I/Os merged with in-queue I/O
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read sectors sectors number of sectors read
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read ticks milliseconds total wait time for read requests
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write I/Os requests number of write I/Os processed
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write merges requests number of write I/Os merged with in-queue I/O
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write sectors sectors number of sectors written
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write ticks milliseconds total wait time for write requests
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in_flight requests number of I/Os currently in flight
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io_ticks milliseconds total time this block device has been active
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time_in_queue milliseconds total wait time for all requests
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discard I/Os requests number of discard I/Os processed
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discard merges requests number of discard I/Os merged with in-queue I/O
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discard sectors sectors number of sectors discarded
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discard ticks milliseconds total wait time for discard requests
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flush I/Os requests number of flush I/Os processed
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flush ticks milliseconds total wait time for flush requests
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=============== ============= =================================================
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read I/Os, write I/Os, discard I/0s
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===================================
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These values increment when an I/O request completes.
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flush I/Os
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==========
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These values increment when an flush I/O request completes.
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Block layer combines flush requests and executes at most one at a time.
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This counts flush requests executed by disk. Not tracked for partitions.
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read merges, write merges, discard merges
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=========================================
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These values increment when an I/O request is merged with an
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already-queued I/O request.
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read sectors, write sectors, discard_sectors
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============================================
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These values count the number of sectors read from, written to, or
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discarded from this block device. The "sectors" in question are the
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standard UNIX 512-byte sectors, not any device- or filesystem-specific
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block size. The counters are incremented when the I/O completes.
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read ticks, write ticks, discard ticks, flush ticks
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===================================================
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These values count the number of milliseconds that I/O requests have
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waited on this block device. If there are multiple I/O requests waiting,
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these values will increase at a rate greater than 1000/second; for
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example, if 60 read requests wait for an average of 30 ms, the read_ticks
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field will increase by 60*30 = 1800.
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in_flight
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=========
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This value counts the number of I/O requests that have been issued to
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the device driver but have not yet completed. It does not include I/O
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requests that are in the queue but not yet issued to the device driver.
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io_ticks
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========
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This value counts the number of milliseconds during which the device has
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had I/O requests queued.
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time_in_queue
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=============
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This value counts the number of milliseconds that I/O requests have waited
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on this block device. If there are multiple I/O requests waiting, this
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value will increase as the product of the number of milliseconds times the
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number of requests waiting (see "read ticks" above for an example).
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