Unaligned writes on block devices
I've read that write of block of unaligned size can induce extra reads.I mean writes on block devices in Linux. Why? How can I see it?
linux block-device
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
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I've read that write of block of unaligned size can induce extra reads.I mean writes on block devices in Linux. Why? How can I see it?
linux block-device
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
add a comment |
I've read that write of block of unaligned size can induce extra reads.I mean writes on block devices in Linux. Why? How can I see it?
linux block-device
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
I've read that write of block of unaligned size can induce extra reads.I mean writes on block devices in Linux. Why? How can I see it?
linux block-device
linux block-device
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
asked Nov 17 '18 at 20:30
GilgameszGilgamesz
1,49411027
1,49411027
add a comment |
add a comment |
1 Answer
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Block devices can only be written or read at their native block alignment, which is for everything I've ever encountered is either 512 bytes or 4096 bytes. You can see what your device's block size is in sysfs:
# cat /sys/block/sdb/queue/logical_block_size
512
Why? The NVMe, SCSI, and ATA command sets simply don't support accessing a region smaller than that. The arguments to the WRITE command are in integer blocks.
If an application needs to write a smaller, or a non-aligned region, then the kernel issues a read to fill in the gaps, then writes out a larger chunk. For example, say you needed to write 256 bytes in the middle of a 512-byte block. The kernel would read the entire 512 bytes from disk, merge your data to write in the correct place, and then write the 512 byte block.
One easy way to monitor reads and writes on your system is with the iostat utility, which is packaged with the sysstat package at least on Centos/RHEL.
[root@bb-cluster-4 md]# iostat -xyz 1
...
avg-cpu: %user %nice %system %iowait %steal %idle
32.29 0.00 7.61 0.00 0.00 60.10
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sdd 0.00 0.00 1.00 0.00 4.00 0.00 8.00 0.00 1.00 1.00 0.00 1.00 0.10
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
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1 Answer
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1 Answer
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oldest
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Block devices can only be written or read at their native block alignment, which is for everything I've ever encountered is either 512 bytes or 4096 bytes. You can see what your device's block size is in sysfs:
# cat /sys/block/sdb/queue/logical_block_size
512
Why? The NVMe, SCSI, and ATA command sets simply don't support accessing a region smaller than that. The arguments to the WRITE command are in integer blocks.
If an application needs to write a smaller, or a non-aligned region, then the kernel issues a read to fill in the gaps, then writes out a larger chunk. For example, say you needed to write 256 bytes in the middle of a 512-byte block. The kernel would read the entire 512 bytes from disk, merge your data to write in the correct place, and then write the 512 byte block.
One easy way to monitor reads and writes on your system is with the iostat utility, which is packaged with the sysstat package at least on Centos/RHEL.
[root@bb-cluster-4 md]# iostat -xyz 1
...
avg-cpu: %user %nice %system %iowait %steal %idle
32.29 0.00 7.61 0.00 0.00 60.10
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sdd 0.00 0.00 1.00 0.00 4.00 0.00 8.00 0.00 1.00 1.00 0.00 1.00 0.10
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
add a comment |
Block devices can only be written or read at their native block alignment, which is for everything I've ever encountered is either 512 bytes or 4096 bytes. You can see what your device's block size is in sysfs:
# cat /sys/block/sdb/queue/logical_block_size
512
Why? The NVMe, SCSI, and ATA command sets simply don't support accessing a region smaller than that. The arguments to the WRITE command are in integer blocks.
If an application needs to write a smaller, or a non-aligned region, then the kernel issues a read to fill in the gaps, then writes out a larger chunk. For example, say you needed to write 256 bytes in the middle of a 512-byte block. The kernel would read the entire 512 bytes from disk, merge your data to write in the correct place, and then write the 512 byte block.
One easy way to monitor reads and writes on your system is with the iostat utility, which is packaged with the sysstat package at least on Centos/RHEL.
[root@bb-cluster-4 md]# iostat -xyz 1
...
avg-cpu: %user %nice %system %iowait %steal %idle
32.29 0.00 7.61 0.00 0.00 60.10
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sdd 0.00 0.00 1.00 0.00 4.00 0.00 8.00 0.00 1.00 1.00 0.00 1.00 0.10
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
add a comment |
Block devices can only be written or read at their native block alignment, which is for everything I've ever encountered is either 512 bytes or 4096 bytes. You can see what your device's block size is in sysfs:
# cat /sys/block/sdb/queue/logical_block_size
512
Why? The NVMe, SCSI, and ATA command sets simply don't support accessing a region smaller than that. The arguments to the WRITE command are in integer blocks.
If an application needs to write a smaller, or a non-aligned region, then the kernel issues a read to fill in the gaps, then writes out a larger chunk. For example, say you needed to write 256 bytes in the middle of a 512-byte block. The kernel would read the entire 512 bytes from disk, merge your data to write in the correct place, and then write the 512 byte block.
One easy way to monitor reads and writes on your system is with the iostat utility, which is packaged with the sysstat package at least on Centos/RHEL.
[root@bb-cluster-4 md]# iostat -xyz 1
...
avg-cpu: %user %nice %system %iowait %steal %idle
32.29 0.00 7.61 0.00 0.00 60.10
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sdd 0.00 0.00 1.00 0.00 4.00 0.00 8.00 0.00 1.00 1.00 0.00 1.00 0.10
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
Block devices can only be written or read at their native block alignment, which is for everything I've ever encountered is either 512 bytes or 4096 bytes. You can see what your device's block size is in sysfs:
# cat /sys/block/sdb/queue/logical_block_size
512
Why? The NVMe, SCSI, and ATA command sets simply don't support accessing a region smaller than that. The arguments to the WRITE command are in integer blocks.
If an application needs to write a smaller, or a non-aligned region, then the kernel issues a read to fill in the gaps, then writes out a larger chunk. For example, say you needed to write 256 bytes in the middle of a 512-byte block. The kernel would read the entire 512 bytes from disk, merge your data to write in the correct place, and then write the 512 byte block.
One easy way to monitor reads and writes on your system is with the iostat utility, which is packaged with the sysstat package at least on Centos/RHEL.
[root@bb-cluster-4 md]# iostat -xyz 1
...
avg-cpu: %user %nice %system %iowait %steal %idle
32.29 0.00 7.61 0.00 0.00 60.10
Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await r_await w_await svctm %util
sdd 0.00 0.00 1.00 0.00 4.00 0.00 8.00 0.00 1.00 1.00 0.00 1.00 0.10
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
answered Dec 3 '18 at 16:52
Mike AndrewsMike Andrews
2,2571226
2,2571226
add a comment |
add a comment |
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