在Proxmox中如果要图形化添加缓存就只能用ZFS了，但是ZFS在实际应用中也有很多问题，所以还是老老实实的用raid10比较稳妥。有些阵列卡是支持cache的（例如我们的存储服务器使用的是H710P就可以设置SSD缓存），但是很可惜我们选用的H310P是不支持的，因此我们需要使用软件进行缓存。
bcache是一个内核级别的缓存软件，在Linux 3.10之后的kernel都已经内置了，只要打开即可。
首先安装bcache的管理软件：
sudo add-apt-repository ppa:g2p/storage sudo apt-get update sudo apt-get install bcache-tools
然后在创建backing盘和cache盘要先格式化磁盘：
root@JS-2002:~# wipefs /dev/sd{a,c}
offset type
----------------------------------------------------------------
0x438 ext4 [filesystem]
UUID: 7f820300-e6e9-46b5-88de-e021adbee990
offset type
----------------------------------------------------------------
0x438 ext4 [filesystem]
UUID: bca5adf5-7036-47d3-be26-74bdda353343
root@JS-2002:~# wipefs /dev/sd{a,c} -a
/dev/sda: 2 bytes were erased at offset 0x00000438 (ext4): 53 ef
/dev/sdc: 2 bytes were erased at offset 0x00000438 (ext4): 53 ef
创建B盘和C盘：
root@JS-2002:~# wipefs /dev/sd{a,c} -a
/dev/sda: 2 bytes were erased at offset 0x00000438 (ext4): 53 ef
/dev/sdc: 2 bytes were erased at offset 0x00000438 (ext4): 53 ef
root@JS-2002:~# make-bcache -B --writeback -w 4KiB /dev/sdc --wipe-bcache
UUID: d0ea10b4-2580-4155-b5f9-dd67f31d5132
Set UUID: 0679b124-73e9-492a-bec0-d1fe2d63858b
version: 1
block_size: 8
data_offset: 16
k /dev/sda --wipe-bcacheche -C -w 4KiB --cache_replacement_policy=lru --writeback
UUID: 1a1ba60f-efc1-4f49-bb8c-68e0cec0c6d0
Set UUID: c03e235e-7b1c-4133-ae5c-bd82a72f4906
version: 0
nbuckets: 1907739
block_size: 8
bucket_size: 1024
nr_in_set: 1
nr_this_dev: 0
first_bucket: 1
我也不知道注册是啥意思，但是文档里说一定要注册B盘和C盘：
echo /dev/sd* > /sys/fs/bcache/register_quiet
这时候就能看到bcache盘了，在/dev/bcache0，然后格式化硬盘并挂载：
mkfs.ext4 /dev/bcache0 mount /dev/bcache0 /mnt
另外如果不小心配置错误的话就去/sys/fs/bcache/路径中对应的UUID路径内，然后执行echo 1 > stop就好了
参考文档原文：
Say you've got a big slow raid 6, and an X-25E or three. Wouldn't it be nice if you could use them as cache... Hence bcache. Wiki and git repositories are at: http://bcache.evilpiepirate.org http://evilpiepirate.org/git/linux-bcache.git http://evilpiepirate.org/git/bcache-tools.git It's designed around the performance characteristics of SSDs - it only allocates in erase block sized buckets, and it uses a hybrid btree/log to track cached extants (which can be anywhere from a single sector to the bucket size). It's designed to avoid random writes at all costs; it fills up an erase block sequentially, then issues a discard before reusing it. Both writethrough and writeback caching are supported. Writeback defaults to off, but can be switched on and off arbitrarily at runtime. Bcache goes to great lengths to protect your data - it reliably handles unclean shutdown. (It doesn't even have a notion of a clean shutdown; bcache simply doesn't return writes as completed until they're on stable storage). Writeback caching can use most of the cache for buffering writes - writing dirty data to the backing device is always done sequentially, scanning from the start to the end of the index. Since random IO is what SSDs excel at, there generally won't be much benefit to caching large sequential IO. Bcache detects sequential IO and skips it; it also keeps a rolling average of the IO sizes per task, and as long as the average is above the cutoff it will skip all IO from that task - instead of caching the first 512k after every seek. Backups and large file copies should thus entirely bypass the cache. In the event of a data IO error on the flash it will try to recover by reading from disk or invalidating cache entries. For unrecoverable errors (meta data or dirty data), caching is automatically disabled; if dirty data was present in the cache it first disables writeback caching and waits for all dirty data to be flushed. Getting started: You'll need make-bcache from the bcache-tools repository. Both the cache device and backing device must be formatted before use. make-bcache -B /dev/sdb make-bcache -C /dev/sdc make-bcache has the ability to format multiple devices at the same time - if you format your backing devices and cache device at the same time, you won't have to manually attach: make-bcache -B /dev/sda /dev/sdb -C /dev/sdc To make bcache devices known to the kernel, echo them to /sys/fs/bcache/register: echo /dev/sdb > /sys/fs/bcache/register echo /dev/sdc > /sys/fs/bcache/register To register your bcache devices automatically, you could add something like this to an init script: echo /dev/sd* > /sys/fs/bcache/register_quiet It'll look for bcache superblocks and ignore everything that doesn't have one. Registering the backing device makes the bcache show up in /dev; you can now format it and use it as normal. But the first time using a new bcache device, it'll be running in passthrough mode until you attach it to a cache. See the section on attaching. The devices show up at /dev/bcacheN, and can be controlled via sysfs from /sys/block/bcacheN/bcache: mkfs.ext4 /dev/bcache0 mount /dev/bcache0 /mnt Cache devices are managed as sets; multiple caches per set isn't supported yet but will allow for mirroring of metadata and dirty data in the future. Your new cache set shows up as /sys/fs/bcache/<UUID> ATTACHING: After your cache device and backing device are registered, the backing device must be attached to your cache set to enable caching. Attaching a backing device to a cache set is done thusly, with the UUID of the cache set in /sys/fs/bcache: echo <UUID> > /sys/block/bcache0/bcache/attach This only has to be done once. The next time you reboot, just reregister all your bcache devices. If a backing device has data in a cache somewhere, the /dev/bcache# device won't be created until the cache shows up - particularly important if you have writeback caching turned on. If you're booting up and your cache device is gone and never coming back, you can force run the backing device: echo 1 > /sys/block/sdb/bcache/running (You need to use /sys/block/sdb (or whatever your backing device is called), not /sys/block/bcache0, because bcache0 doesn't exist yet. If you're using a partition, the bcache directory would be at /sys/block/sdb/sdb2/bcache) The backing device will still use that cache set if it shows up in the future, but all the cached data will be invalidated. If there was dirty data in the cache, don't expect the filesystem to be recoverable - you will have massive filesystem corruption, though ext4's fsck does work miracles. ERROR HANDLING: Bcache tries to transparently handle IO errors to/from the cache device without affecting normal operation; if it sees too many errors (the threshold is configurable, and defaults to 0) it shuts down the cache device and switches all the backing devices to passthrough mode. - For reads from the cache, if they error we just retry the read from the backing device. - For writethrough writes, if the write to the cache errors we just switch to invalidating the data at that lba in the cache (i.e. the same thing we do for a write that bypasses the cache) - For writeback writes, we currently pass that error back up to the filesystem/userspace. This could be improved - we could retry it as a write that skips the cache so we don't have to error the write. - When we detach, we first try to flush any dirty data (if we were running in writeback mode). It currently doesn't do anything intelligent if it fails to read some of the dirty data, though. TROUBLESHOOTING PERFORMANCE: Bcache has a bunch of config options and tunables. The defaults are intended to be reasonable for typical desktop and server workloads, but they're not what you want for getting the best possible numbers when benchmarking. - Bad write performance If write performance is not what you expected, you probably wanted to be running in writeback mode, which isn't the default (not due to a lack of maturity, but simply because in writeback mode you'll lose data if something happens to your SSD) # echo writeback > /sys/block/bcache0/cache_mode - Bad performance, or traffic not going to the SSD that you'd expect By default, bcache doesn't cache everything. It tries to skip sequential IO - because you really want to be caching the random IO, and if you copy a 10 gigabyte file you probably don't want that pushing 10 gigabytes of randomly accessed data out of your cache. But if you want to benchmark reads from cache, and you start out with fio writing an 8 gigabyte test file - so you want to disable that. # echo 0 > /sys/block/bcache0/bcache/sequential_cutoff To set it back to the default (4 mb), do # echo 4M > /sys/block/bcache0/bcache/sequential_cutoff - Traffic's still going to the spindle/still getting cache misses In the real world, SSDs don't always keep up with disks - particularly with slower SSDs, many disks being cached by one SSD, or mostly sequential IO. So you want to avoid being bottlenecked by the SSD and having it slow everything down. To avoid that bcache tracks latency to the cache device, and gradually throttles traffic if the latency exceeds a threshold (it does this by cranking down the sequential bypass). You can disable this if you need to by setting the thresholds to 0: # echo 0 > /sys/fs/bcache/<cache set>/congested_read_threshold_us # echo 0 > /sys/fs/bcache/<cache set>/congested_write_threshold_us The default is 2000 us (2 milliseconds) for reads, and 20000 for writes. - Still getting cache misses, of the same data One last issue that sometimes trips people up is actually an old bug, due to the way cache coherency is handled for cache misses. If a btree node is full, a cache miss won't be able to insert a key for the new data and the data won't be written to the cache. In practice this isn't an issue because as soon as a write comes along it'll cause the btree node to be split, and you need almost no write traffic for this to not show up enough to be noticable (especially since bcache's btree nodes are huge and index large regions of the device). But when you're benchmarking, if you're trying to warm the cache by reading a bunch of data and there's no other traffic - that can be a problem. Solution: warm the cache by doing writes, or use the testing branch (there's a fix for the issue there). SYSFS - BACKING DEVICE: attach Echo the UUID of a cache set to this file to enable caching. cache_mode Can be one of either writethrough, writeback, writearound or none. clear_stats Writing to this file resets the running total stats (not the day/hour/5 minute decaying versions). detach Write to this file to detach from a cache set. If there is dirty data in the cache, it will be flushed first. dirty_data Amount of dirty data for this backing device in the cache. Continuously updated unlike the cache set's version, but may be slightly off. label Name of underlying device. readahead Size of readahead that should be performed. Defaults to 0. If set to e.g. 1M, it will round cache miss reads up to that size, but without overlapping existing cache entries. running 1 if bcache is running (i.e. whether the /dev/bcache device exists, whether it's in passthrough mode or caching). sequential_cutoff A sequential IO will bypass the cache once it passes this threshhold; the most recent 128 IOs are tracked so sequential IO can be detected even when it isn't all done at once. sequential_merge If non zero, bcache keeps a list of the last 128 requests submitted to compare against all new requests to determine which new requests are sequential continuations of previous requests for the purpose of determining sequential cutoff. This is necessary if the sequential cutoff value is greater than the maximum acceptable sequential size for any single request. state The backing device can be in one of four different states: no cache: Has never been attached to a cache set. clean: Part of a cache set, and there is no cached dirty data. dirty: Part of a cache set, and there is cached dirty data. inconsistent: The backing device was forcibly run by the user when there was dirty data cached but the cache set was unavailable; whatever data was on the backing device has likely been corrupted. stop Write to this file to shut down the bcache device and close the backing device. writeback_delay When dirty data is written to the cache and it previously did not contain any, waits some number of seconds before initiating writeback. Defaults to 30. writeback_percent If nonzero, bcache tries to keep around this percentage of the cache dirty by throttling background writeback and using a PD controller to smoothly adjust the rate. writeback_rate Rate in sectors per second - if writeback_percent is nonzero, background writeback is throttled to this rate. Continuously adjusted by bcache but may also be set by the user. writeback_running If off, writeback of dirty data will not take place at all. Dirty data will still be added to the cache until it is mostly full; only meant for benchmarking. Defaults to on. SYSFS - BACKING DEVICE STATS: There are directories with these numbers for a running total, as well as versions that decay over the past day, hour and 5 minutes; they're also aggregated in the cache set directory as well. bypassed Amount of IO (both reads and writes) that has bypassed the cache cache_hits cache_misses cache_hit_ratio Hits and misses are counted per individual IO as bcache sees them; a partial hit is counted as a miss. cache_bypass_hits cache_bypass_misses Hits and misses for IO that is intended to skip the cache are still counted, but broken out here. cache_miss_collisions Counts instances where data was going to be inserted into the cache from a cache miss, but raced with a write and data was already present (usually 0 since the synchronization for cache misses was rewritten) cache_readaheads Count of times readahead occured. SYSFS - CACHE SET: average_key_size Average data per key in the btree. bdev<0..n> Symlink to each of the attached backing devices. block_size Block size of the cache devices. btree_cache_size Amount of memory currently used by the btree cache bucket_size Size of buckets cache<0..n> Symlink to each of the cache devices comprising this cache set. cache_available_percent Percentage of cache device which doesn't contain dirty data, and could potentially be used for writeback. This doesn't mean this space isn't used for clean cached data; the unused statistic (in priority_stats) is typically much lower. clear_stats Clears the statistics associated with this cache dirty_data Amount of dirty data is in the cache (updated when garbage collection runs). flash_vol_create Echoing a size to this file (in human readable units, k/M/G) creates a thinly provisioned volume backed by the cache set. io_error_halflife io_error_limit These determines how many errors we accept before disabling the cache. Each error is decayed by the half life (in # ios). If the decaying count reaches io_error_limit dirty data is written out and the cache is disabled. journal_delay_ms Journal writes will delay for up to this many milliseconds, unless a cache flush happens sooner. Defaults to 100. root_usage_percent Percentage of the root btree node in use. If this gets too high the node will split, increasing the tree depth. stop Write to this file to shut down the cache set - waits until all attached backing devices have been shut down. tree_depth Depth of the btree (A single node btree has depth 0). unregister Detaches all backing devices and closes the cache devices; if dirty data is present it will disable writeback caching and wait for it to be flushed. SYSFS - CACHE SET INTERNAL: This directory also exposes timings for a number of internal operations, with separate files for average duration, average frequency, last occurence and max duration: garbage collection, btree read, btree node sorts and btree splits. active_journal_entries Number of journal entries that are newer than the index. btree_nodes Total nodes in the btree. btree_used_percent Average fraction of btree in use. bset_tree_stats Statistics about the auxiliary search trees btree_cache_max_chain Longest chain in the btree node cache's hash table cache_read_races Counts instances where while data was being read from the cache, the bucket was reused and invalidated - i.e. where the pointer was stale after the read completed. When this occurs the data is reread from the backing device. trigger_gc Writing to this file forces garbage collection to run. SYSFS - CACHE DEVICE: block_size Minimum granularity of writes - should match hardware sector size. btree_written Sum of all btree writes, in (kilo/mega/giga) bytes bucket_size Size of buckets cache_replacement_policy One of either lru, fifo or random. discard Boolean; if on a discard/TRIM will be issued to each bucket before it is reused. Defaults to off, since SATA TRIM is an unqueued command (and thus slow). freelist_percent Size of the freelist as a percentage of nbuckets. Can be written to to increase the number of buckets kept on the freelist, which lets you artificially reduce the size of the cache at runtime. Mostly for testing purposes (i.e. testing how different size caches affect your hit rate), but since buckets are discarded when they move on to the freelist will also make the SSD's garbage collection easier by effectively giving it more reserved space. io_errors Number of errors that have occured, decayed by io_error_halflife. metadata_written Sum of all non data writes (btree writes and all other metadata). nbuckets Total buckets in this cache priority_stats Statistics about how recently data in the cache has been accessed. This can reveal your working set size. Unused is the percentage of the cache that doesn't contain any data. Metadata is bcache's metadata overhead. Average is the average priority of cache buckets. Next is a list of quantiles with the priority threshold of each. written Sum of all data that has been written to the cache; comparison with btree_written gives the amount of write inflation in bcache.
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