bcache: documentation formatting, edited for clarity, stripe alignment notes
Signed-off-by: Eric Wheeler <bcache@linux.ewheeler.net> Cc: Marc MERLIN <marc@merlins.org> Signed-off-by: Jonathan Corbet <corbet@lwn.net>steinar/wifi_calib_4_9_kernel
Eric Wheeler
Jonathan Corbet
parent
c9b2ffc022
commit
c0b8c9a344
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@ -137,48 +137,60 @@ the backing devices to passthrough mode.
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HOWTO/COOKBOOK
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--------------
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A) Your bcache doesn't start.
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Starting and starting a bcache with a missing caching device
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A) Starting a bcache with a missing caching device
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Registering the backing device doesn't help, it's already there, you just need
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If registering the backing device doesn't help, it's already there, you just need
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to force it to run without the cache:
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host:~# echo /dev/sdb1 > /sys/fs/bcache/register
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[ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
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host:~# echo /dev/sdb1 > /sys/fs/bcache/register
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[ 119.844831] bcache: register_bcache() error opening /dev/sdb1: device already registered
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Next, you try to register your caching device if it's present. However if it's
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absent, or registration fails for some reason, you can still start your bcache
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without its cache, like so:
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host:/sys/block/sdb/sdb1/bcache# echo 1 > running
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Next, you try to register your caching device if it's present. However
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if it's absent, or registration fails for some reason, you can still
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start your bcache without its cache, like so:
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host:/sys/block/sdb/sdb1/bcache# echo 1 > running
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Note that this may cause data loss if you were running in writeback mode.
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B) Bcache not finding its cache and not starting
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B) Bcache does not find its cache
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This does not work:
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host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
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[ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
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[ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
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[ 1933.478179] : cache set not found
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host:/sys/block/md5/bcache# echo 0226553a-37cf-41d5-b3ce-8b1e944543a8 > attach
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[ 1933.455082] bcache: bch_cached_dev_attach() Couldn't find uuid for md5 in set
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[ 1933.478179] bcache: __cached_dev_store() Can't attach 0226553a-37cf-41d5-b3ce-8b1e944543a8
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[ 1933.478179] : cache set not found
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In this case, the caching device was simply not registered at boot or
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disappeared and came back, and needs to be (re-)registered:
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host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
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In this case, the caching device was simply not registered at boot
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or disappeared and came back, and needs to be (re-)registered:
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host:/sys/block/md5/bcache# echo /dev/sdh2 > /sys/fs/bcache/register
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C) Corrupt bcache caching device crashes the kernel on startup/boot
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C) Corrupt bcache crashes the kernel at device registration time:
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You'll have to wipe the caching device, start the backing device without the
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cache, and you can re-attach the cleaned up caching device then. This does
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require booting with a kernel/rescue media where bcache is disabled
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since it will otherwise try to access your device and probably crash
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again before you have a chance to wipe it.
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(or if you plan ahead, compile a backup kernel with bcache disabled and keep it
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in your grub config for a rainy day)
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If bcache is not available in the kernel, a filesystem on the backing device is
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still available at an 8KiB offset. So either via a loopdev of the backing device
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created with --offset 8K or by temporarily increasing the start sector of the
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partition by 16 (512byte sectors).
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This should never happen. If it does happen, then you have found a bug!
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Please report it to the bcache development list: linux-bcache@vger.kernel.org
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Be sure to provide as much information that you can including kernel dmesg
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output if available so that we may assist.
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D) Recovering data without bcache:
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If bcache is not available in the kernel, a filesystem on the backing
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device is still available at an 8KiB offset. So either via a loopdev
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of the backing device created with --offset 8K, or any value defined by
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--data-offset when you originally formatted bcache with `make-bcache`.
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For example:
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losetup -o 8192 /dev/loop0 /dev/your_bcache_backing_dev
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This should present your unmodified backing device data in /dev/loop0
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If your cache is in writethrough mode, then you can safely discard the
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cache device without loosing data.
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E) Wiping a cache device
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This is how you wipe the caching device:
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host:~# wipefs -a /dev/sdh2
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16 bytes were erased at offset 0x1018 (bcache)
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they were: c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
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@ -205,56 +217,60 @@ host:/sys/block/md5/bcache# echo 5bc072a8-ab17-446d-9744-e247949913c1 > attach
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[ 865.276616] bcache: bch_cached_dev_attach() Caching md5 as bcache0 on set 5bc072a8-ab17-446d-9744-e247949913c1
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D) Remove or replace a caching device
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F) Remove or replace a caching device
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host:/sys/block/sda/sda7/bcache# echo 1 > detach
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[ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
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host:/sys/block/sda/sda7/bcache# echo 1 > detach
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[ 695.872542] bcache: cached_dev_detach_finish() Caching disabled for sda7
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host:~# wipefs -a /dev/nvme0n1p4
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wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
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Ooops, it's disabled, but not unregistered, so it's still protected
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host:~# wipefs -a /dev/nvme0n1p4
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wipefs: error: /dev/nvme0n1p4: probing initialization failed: Device or resource busy
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Ooops, it's disabled, but not unregistered, so it's still protected
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We need to go and unregister it:
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host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
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lrwxrwxrwx 1 root root 0 Feb 25 18:33 cache0 -> ../../../devices/pci0000:00/0000:00:1d.0/0000:70:00.0/nvme/nvme0/nvme0n1/nvme0n1p4/bcache/
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host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
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kernel: [ 917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
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host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# ls -l cache0
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lrwxrwxrwx 1 root root 0 Feb 25 18:33 cache0 -> ../../../devices/pci0000:00/0000:00:1d.0/0000:70:00.0/nvme/nvme0/nvme0n1/nvme0n1p4/bcache/
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host:/sys/fs/bcache/b7ba27a1-2398-4649-8ae3-0959f57ba128# echo 1 > stop
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kernel: [ 917.041908] bcache: cache_set_free() Cache set b7ba27a1-2398-4649-8ae3-0959f57ba128 unregistered
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Now we can wipe it:
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host:~# wipefs -a /dev/nvme0n1p4
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/dev/nvme0n1p4: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
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host:~# wipefs -a /dev/nvme0n1p4
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/dev/nvme0n1p4: 16 bytes were erased at offset 0x00001018 (bcache): c6 85 73 f6 4e 1a 45 ca 82 65 f5 7f 48 ba 6d 81
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E) dmcrypt and bcache
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G) dm-crypt and bcache
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First setup bcache unencrypted and then install dmcrypt on top of /dev/bcache<N>
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This will work faster than if you dmcrypt both the backing and caching
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devices and then install bcache on top.
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First setup bcache unencrypted and then install dmcrypt on top of
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/dev/bcache<N> This will work faster than if you dmcrypt both the backing
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and caching devices and then install bcache on top. [benchmarks?]
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F) Stop/free a registered bcache to wipe and/or recreate it
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(or maybe you need to free up all bcache references so that you can have fdisk
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run and re-register a changed partition table, which won't work if there are any
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active backing or caching devices left on it)
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H) Stop/free a registered bcache to wipe and/or recreate it
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Suppose that you need to free up all bcache references so that you can
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fdisk run and re-register a changed partition table, which won't work
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if there are any active backing or caching devices left on it:
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1) Is it present in /dev/bcache* ? (there are times where it won't be)
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If so, it's easy:
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host:/sys/block/bcache0/bcache# echo 1 > stop
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host:/sys/block/bcache0/bcache# echo 1 > stop
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2) But if your backing device is gone, this won't work:
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host:/sys/block/bcache0# cd bcache
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bash: cd: bcache: No such file or directory
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host:/sys/block/bcache0# cd bcache
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bash: cd: bcache: No such file or directory
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In this case, you may have to unregister the dmcrypt block device that
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references this bcache to free it up:
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host:~# dmsetup remove oldds1
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bcache: bcache_device_free() bcache0 stopped
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bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
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host:~# dmsetup remove oldds1
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bcache: bcache_device_free() bcache0 stopped
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bcache: cache_set_free() Cache set 5bc072a8-ab17-446d-9744-e247949913c1 unregistered
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This causes the backing bcache to be removed from /sys/fs/bcache and then it can
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be reused
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This causes the backing bcache to be removed from /sys/fs/bcache and
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then it can be reused. This would be true of any block device stacking
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where bcache is a lower device.
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3) In other cases, you can also look in /sys/fs/bcache/:
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host:/sys/fs/bcache# ls -l */{cache?,bdev?}
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lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/bdev1 -> ../../../devices/virtual/block/dm-1/bcache/
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lrwxrwxrwx 1 root root 0 Mar 5 09:39 0226553a-37cf-41d5-b3ce-8b1e944543a8/cache0 -> ../../../devices/virtual/block/dm-4/bcache/
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@ -262,9 +278,10 @@ lrwxrwxrwx 1 root root 0 Mar 5 09:39 5bc072a8-ab17-446d-9744-e247949913c1/cache
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The device names will show which UUID is relevant, cd in that directory
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and stop the cache:
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host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
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this will free up bcache references and let you reuse the partition for other
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purposes.
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host:/sys/fs/bcache/5bc072a8-ab17-446d-9744-e247949913c1# echo 1 > stop
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This will free up bcache references and let you reuse the partition for
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other purposes.
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@ -275,6 +292,22 @@ Bcache has a bunch of config options and tunables. The defaults are intended to
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be reasonable for typical desktop and server workloads, but they're not what you
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want for getting the best possible numbers when benchmarking.
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- Backing device alignment
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The default metadata size in bcache is 8k. If your backing device is
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RAID based, then be sure to align this by a multiple of your stride
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width using `make-bcache --data-offset`. If you intend to expand your
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disk array in the future, then multiply a series of primes by your
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raid stripe size to get the disk multiples that you would like.
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For example: If you have a 64k stripe size, then the following offset
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would provide alignment for many common RAID5 data spindle counts:
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64k * 2*2*2*3*3*5*7 bytes = 161280k
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That space is wasted, but for only 157.5MB you can grow your RAID 5
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volume to the following data-spindle counts without re-aligning:
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3,4,5,6,7,8,9,10,12,14,15,18,20,21 ...
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- Bad write performance
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If write performance is not what you expected, you probably wanted to be
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@ -382,7 +415,7 @@ sequential_merge
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against all new requests to determine which new requests are sequential
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continuations of previous requests for the purpose of determining sequential
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cutoff. This is necessary if the sequential cutoff value is greater than the
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maximum acceptable sequential size for any single request.
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maximum acceptable sequential size for any single request.
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state
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The backing device can be in one of four different states:
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@ -469,7 +502,7 @@ bucket_size
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Size of buckets
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cache<0..n>
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Symlink to each of the cache devices comprising this cache set.
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Symlink to each of the cache devices comprising this cache set.
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cache_available_percent
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Percentage of cache device which doesn't contain dirty data, and could
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