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Slow writes to disk

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

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?