Ubuntu – BCache and disk encryption

bcachelinuxluksUbuntu

I recently bought a new laptop with a 16Gb mSATA SSD cache drive. I haven't used that one yet.

I have, however, opted for Ubuntu 13.04 with "Full Disk Encryption" for the main partition (is that what's called LUKS?).

With bcache making it's way into 3.10, I'd like to take advantage of the aforementioned cache drive.

According to this, one has to format one's backing and cache drives in order to take advantage of bcache.

My questions are:

Which order do I proceed in? Set up bcache, then (re)setup LUKS or the other way around?
Except for the few files pertaining to the encrypted setup (incl. /etc/fstab), can I tar/rsync/whatever the rest of the existing fs to another disk, set up bcache and LUKS and then tar/rsync/whatever back and expect things to work?
Are there other things I should know about?

Best Answer

To answer 1., the most sensible thing to do is to put bcache on top of two LUKS virtual devices. LUKS-encrypting a bcache device might work, but there's no guarantee LUKS will consistently put the same virtual sector in the same physical sector every time. You can encrypt both LUKS devices with the same keyfile and unlock both at the same time.

Related Solutions

Linux – bcache on md or md on bcache

I think caching the whole md device make most sense.

Putting bcache to cache the whole md device sacrifices the whole idea of having raid, because it introduces another single point of failure.

OTH failurs of SSD disks are relatively rare, and bcache can be put into the writethrough/writearound mode (in contrast to the writeback mode), where there is no data stored only to the cache device, and failure of the cache doesn't kill the information in the raid makes it a relatively safe option.
Other fact is that there is significant computational overhead of soft RAID-5; when caching each spinning raid member separately, computer still has to re-calculate all the parities, even on cache hits
~~Obviously, you'd sacrifice some expensive ssd space, if you cache each spinning drive separately.~~ - Unless you plan to use raided ssd cache.
Both options relatively don't affect the time of growing process - although the option with spinning drives being cached separately has potential to be slower due to more bus traffic.

It is fast and relatively simple process to configure bcache to remove the ssd drive, when you need to replace it. Thanks to the blocks it should be possible to migrate the raid setup both ways on-place.

You should also remember, that at the moment most (all?) live-CD distributions don't support bcache, so you can't simply access your data with such tools regardless of the bcache-mdraid layout option you chose.

Optimizing bcache

I have same problem. My disk IO still bypass by bcache. After set congested_read_threshold_us and congested_write_threshold_us following bcache documentation. My problem is solved.

- 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.

All disk IO are sent to my SSD(sde) now.

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sdb               0.00     0.00    0.00    0.30     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
sdd               0.00     0.00    0.10    0.30     0.80     0.00     4.00     0.00    3.00   12.00    0.00   3.00   0.12
sdc               0.00     0.00    2.20    0.30    26.00     0.00    20.80     0.00    1.76    2.00    0.00   1.76   0.44
sda               0.00     0.00    0.20    0.20     0.80     0.00     4.00     0.01    8.00   16.00    0.00  13.00   0.52
sde               0.00   293.20   81.70  232.70  1129.20 58220.00   377.54     6.62   21.05   27.69   18.71   3.18 100.00
md1               0.00     0.00    2.50    0.30    27.60     0.00    19.71     0.00    0.00    0.00    0.00   0.00   0.00
md0               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00
bcache0           0.00     0.00   83.00  402.40  1156.80 28994.80   124.23    15.36   31.70   27.02   32.67   2.06  99.92

Best Answer

Related Solutions

Linux – bcache on md or md on bcache

Optimizing bcache

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