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I have set up /dev/sda3 on my SSD to cache /dev/sdb3 on my HDD as /dev/bcache0 using bcache in Ubuntu 13.10 (kernel 3.11.0-14-generic). I have formatted it as ext4.

I currently mount / (on my SSD) and /home (bcache) using these fstab options

# / (root) -> /dev/sda2 (SSD)
UUID={uuid for /dev/sda2}   /  ext4  discard,noatime,nodiratime,errors=remount-ro   0  1

# /home -> { /dev/bcache0 -> { /dev/sda3 -> /dev/sdb3 } }
UUID={uuid for /dev/bcache0}  /home  ext4  defaults   0  2

What I am wondering is if any mount options (e.g. discard (TRIM), noatime, etc) are useful when mounting bcache, or if bcache its self takes care of optimising the filesystem for the SSD and HDD partitions.

Please let me know if I have not made my question clear, thanks.

MattSturgeon
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3 Answers3

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discard is a special case: bcache will (I think) make use of it to discard sectors, but won't pass it down the stack unless you have explicitly asked it to (echo 1 |sudo tee -a /sys/block/*/bcache/discard, which will be remembered across reboots). The best case for performance is that your filesystem sends discards (which means you need to put the flag in /etc/fstab) and bcache passes them down if and only if your SSD is SATA 3.1 (queued TRIM support).

Your filesystem is able to detect bcache as something like an SSD (rotational=0 in sysfs), which it may use to tune itself a bit differently. Aside from discard and noatime, I don't think there's a real need to stray from ext4 defaults.

Gabriel
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  • I think what I want is for bcache to use `discard` and `noatime` on the SSD cache, but not on the HDD storage partition. Are you saying that by mounting the `ext4` filesystem with those options and telling bcache to pass them down to each device, they will only be applied to applicable devices? Or will bcache try to use `discard` on my HDD as well? Or do I not need bcache to pass them down at all to get the benefits on the SSD cache? Apologies for my confusion. – MattSturgeon Jan 12 '14 at 14:12
  • If you want bcache to discard buckets on the SSD, you must use bcache's sysfs flag. I'm not sure about the rest; it would take more familiarity with the code. – Gabriel Jan 12 '14 at 23:42
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If you put discard in the fstab mount arguments for a filesystem on a bcache device, then the filesystem will issue TRIM commands to bcache for deleted blocks. In theory bcache could use incoming TRIM commands as hints for freeing cached data, but I don't think it does, and there are hints searching online that bcache doesn't understand incoming TRIM commands at all (I found one unverified report of errors).

However, bcache can itself send TRIM commands when it deletes buckets on its caching SSD device. By default it doesn't enable this because TRIM is not queued on pre-SATA3.1 drives so it's slower, but on modern SSDs enabling discard gives better peformance.

TLDR; don't put "discard" in /etc/fstab for a filesystem on a bcache device, but endable bcache's discard in /sys/block/*/bcache/discard if your cache device is a modern SATA3.1 SSD.

UPDATE 2023-02-28: For at least kernel 6.x.x things have changed.

For bcache the /sys/block/*/bcache/discard setting doesn't exist any more, at least for my old ssd. It might only appear for drives that support queued trim (SATA3.1) or just defaults to "on" for drives that support it. It doesn't seem to have any problem accepting TRIM operations from filesystems, but I don't know if it just ignores them or uses them as hints for discarding buckets.

For btrfs it automatically detects bcache devices as "non-rotational" so adds the ssd mount option by default. It will "discard" whole bcache devices when they are first added, but discard is not "on" by default. I suspect using the btrfs mount option discard=async would be fine; worst case bcache ignores it, best case it uses it as a hint for deleting buckets.

Another btrfs mount option that is probably worth adding is ssd_spread. From the description this turns on trying to keep data contiguous. It currently defaults to off because modern ssd's don't care, but it's probably beneficial for bcache as it will keep data more contiguous on the non-ssd backing drives.

Donovan Baarda
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  • I think this is now old advice. For starters `/sys/block/*/bcache/discard` doesn't exist any more with kernel 6.1.0 and I can't find an equivalent, which suggests it's always on unless there's some kind of drive blacklist for TRIM support. Also my btrfs filesystem is not explicitly mounted with `discard` enabled but it's definitely issuing discards to the bcache devices and it seems to be handling them just fine. – Donovan Baarda Feb 27 '23 at 22:09
  • Correction; btrfs is not using automatically using discard on my bcache devices without specific mount options for it, but `btrfs device add ...` did discard the whole bcache device first before adding it. btrfs is automatically using it's `ssd` mount option for bcache devices, but this does not turn on discard. I've updated my answer to include these updates. – Donovan Baarda Feb 28 '23 at 01:52
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On Kernel 6.4.11 I've managed to find discard for my WD SN750: /sys/block/nvme0n1/nvme0n1p1/bcache/discard

It is disabled by default

Francisco Boni Neto
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