On Fri, 2012-10-12 at 22:34 +0200, Milan Broz wrote:
> On 10/12/2012 09:37 PM, Michael Zugelder wrote:
> > Testing setup:
> > * Fedora 17, stock 3.5.4-2.fc17 kernel and a self-compiled 3.6.1 kernel
> > * 320 GiB USB hard drive (sdb)
> I guess that USB is the key factor here... I remember to have similar
> problem some time ago even without dmcrypt.
> Is it reproducible with the same kernel cfg but with internal disk?
I noticed this problem on my encrypted root partition and used the
USB device to reproduce it. It's just much easier to get writeback on a
simple 20 MiB/s device and being able to actually use the root device
while performing tests.
My root device (SATA2, Samsung SSD 830, aes-xts-plain, btrfs):
3463 seeks/second, 0.29 ms random access time
0 seeks/second, 4285.71 ms random access time
> You can also test completely fake underlying device,
> use device-mapper- zero target:
> dmsetup create dev_zero --table "0 <sectors size> zero"
> (All writes are dropped and all reads returns zero in this case.)
> Is there any starvation with this setup? (It shouldn't.)
Using the zero target alone, no issues (192286 seeks/second).
> Btw you can use cryptsetup with cipher "null" to simplify
> (I added to cryptsetup to test exactly such scenarios).
Neat, but doesn't work with the device mapper null target. Using raw
dmsetup with crypto_null results in a nice test case:
For some time period, reads are fine (see the relatively low average
random access time), but sometimes reads take multiple seconds. A
benchmark showing min/max/avg/med/stdev values for random reads would be
> > * writeback induced by running 'dd if=/dev/zero of=$target bs=1M'
> Any change if you use oflag=direct ? (iow using direct io)
No issues while using direct IO (25054 seeks/second, no obvious spikes)
using the null target, cipher_null test from above.
> > I experimented a bit with the other device mapper targets, namely linear
> > and stripe, but both worked completely fine. I also tried putting a
> > linear mapping above dm-crypt, with no impact on performance. Comparing
> > the content of the /sys/block/$DEV files of the linear mapping and
> > dm-crypt, there are no differences beside the name, dev no, stats,
> > uevent and inflight files.
> There is crucial difference between linear/stripe and dmcrypt:
> linear just remaps IO target device, dmcrypt queues operations
> (using kernel workqueue) and creates full bio clones.
> So comparison here is IMHO not much helpful.
Okay, I just wanted to rule out a general device mapper problem.
> There are two internal dmcrypt queues, but I think that the problem
> is triggered by some combination with USB storage backend.
Results above seem to indicate otherwise.
> > Any pointers would be appreciated, I haven't found much on the web about
> > this issue.
> Btw there was a proposed rewrite of internal dmcrypt queues, if you have time,
> you can try if it changes anything for your use case.
> Patches in dm-devel archive
Seems interesting, I'll try it out tomorrow.
dm-devel mailing list
10-13-2012, 08:56 PM
PROBLEM: read starvation during writeback
I have a small update on my previously posted test case. I noticed
'cryptsetup -c null' uses cipher_null in ecb mode, and in that case, I
don't observe any read starvation. I'm not sure what the default cipher
mode is (cryptsetup uses cbc-essiv:sha256), but I can reproduce the
problem using just 'cipher_null-cbc-plain'.
Note that for some reason, dd writes extremely slow (below 100.0 MB/s on
my machine) in this test.
On the other hand, cipher_null-ecb works fine.
# dd if=/dev/zero bs=1M of=/dev/mapper/cipher_null-ecb
# ioping -R /dev/mapper/cipher_null-ecb
32337 requests completed in 3000.0 ms, 54042 iops, 211.1 mb/s
min/avg/max/mdev = 0.0/0.0/18.8/0.2 ms
dd writes at around 850 MB/s in that case (1.8 GB/s directly to the null
I tried similar benchmarks using aes instead of cipher_null, but found
no bothersome spikes with aes-ecb, aes-cbc-plain, aes-cbc-essiv:sha256.
But aes-xts-plain sometimes drops down to 5 iops (over the course of 3
So there is probably something very wrong with cipher_null-cbc-plain
being an order of magnitude slower than aes-cbc-plain, but the cbc mode
itself doesn't seem to cause the problem.
I also ran the benchmarks on an old Athlon 64 X2 3800+ box running
kernel 3.2 and could reproduce it with very first try for every cipher
spec I tried (cipher_null-cbc-plain, cipher_null-ecb, aes-cbc-plain,
aes-ecb, aes-xts-plain). Best I could achieve was 5 iops. But
interestingly, I didn't observe the really huge spikes (upwards of 20
seconds), as I do on my 1nd gen Core i5 Notebook.
On Sat, 2012-10-13 at 01:06 +0200, Michael Zugelder wrote:
> On Fri, 2012-10-12 at 22:34 +0200, Milan Broz wrote:
> > Btw there was a proposed rewrite of internal dmcrypt queues, if you have time,
> > you can try if it changes anything for your use case.
> > Patches in dm-devel archive
> > http://www.redhat.com/archives/dm-devel/2012-August/msg00210.html
> Seems interesting, I'll try it out tomorrow.
I tried the patch set earlier today, but had the same issues when
reading while writing to a 'crypt_null' mapping.
Anything else I could do to diagnose this problem?
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