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Re: Slow writes to disk




On 11/2/18 5:31 AM, Stefan Monnier wrote:
For a while now I noticed that aptitude is very slow on one of my
machine (Thinkpad T61) running Debian testing.  At first I thought it
was because its disk (a fairly old 120GB SSD) was suffering from some
kind of problem, so I replaced it with an almost new 240GB Samsung 840.

It seemed to bet better at first, but maybe it was just an impression.
In any case, now it's definitely very slow.  Digging more into it,
I found out that part of the problem seems to be very slow writes to
the disk.  I can reproduce tests where `dd`ing a 40MB file proceeds at
the ridiculous speed of about 500KB/s (tho sometimes the speed is much
higher).

I've been using SSD drives on most of my machines for many years now,
and it's the first time I notice such a slow performance.  `smartctl`
shows that the drive an not registered any errors at all and has a "wear
leveling" of 18.

I figured maybe the drive is having trouble for lack of free space which
leads to high load on the "GC", so I did a `fstrim` on / and /home
(both on LVM), which took a bit of time and returned without any
message, which I took to be a positive sign that it managed to do what
it's supposed to.  But it made no difference to the performance.

Any idea?

1.  Backup your data and configuration settings.

2. Run the following commands as root and paste your console session (prompt, commands, output) into a reply:

	cat /etc/debian_version

	uname -a

	lsblk

	parted -l

	mount

	df	

For the /dev/sdX that corresponds to the "240 GB Samsung 840" (replace sdX with the correct letter):

	smartctl -i /dev/sdX

3. Download and run the manufacturer's diagnostic utility (Windows may be required):

	https://www.samsung.com/us/support/owners/product/840-evo-sata-iii

If the tool reports the drive is bad, RMA or recycle it.

4. Most likely, the manufacturer diagnostic will say the drive is good. This means the the problem is software -- e.g. bugs, misconfiguration, incompatible packages, cruft, gremlins, etc.. Find a disk benchmarking tool. I use dbench:

	https://linux.die.net/man/1/dbench

Install and run it.  Post the summary at the end.  For example:

	dbench -D /mnt/sda4 10 | tee dbench-i72600s-debian-7-amd64-btrfs.out
	...
	 Operation      Count    AvgLat    MaxLat
	 ----------------------------------------
	 NTCreateX    23414985     0.018    21.010
	 Close        17200117     0.001     2.229
	 Rename        991488     0.116    21.193
	 Unlink       4728380     0.093    21.652
	 Deltree          600     5.621    22.280
	 Mkdir            300     0.003     0.024
	 Qpathinfo    21223225     0.009     6.276
	 Qfileinfo    3719536     0.001     2.636
	 Qfsinfo      3891488     0.005     2.656
	 Sfileinfo    1907346     0.006     8.431
	 Find         8205416     0.034    12.780
	 WriteX       11675714     0.018    20.792
	 ReadX        36703998     0.004     8.348
	 LockX          76244     0.003     0.302
	 UnlockX        76244     0.002     0.276
	 Flush        1641062     2.368    51.611

	Throughput 1225.6 MB/sec  10 clients  10 procs  max_latency=51.617 ms

Look in dmesg and /var/log/* for errors, warnings, or other clues. Post anything that looks useful.


David