Does SCSI eat my PC's memory? Probably not, but I can't find an
explanation of what uses so much RAM when I use SCSI disks.
A little background first.
Overall, the system has 1 GB memory. When it boots and everything has
started, about 60 MB of memory is used (excluding buffers).
One partition is on a iSCSI target (I access it using open-iscsi):
/dev/sda 570G 402G 139G 75% /mnt/iscsi_backup
It contains lots of data, many files, hardlinked multiple times
(in total, almost 100 000 000 files).
When I run "find /mnt/iscsi_backup" for some time, and run
"free" to see memory usage, I can see almost 500 MB is used (excluding
buffers/cache):
# free
total used free shared buffers cached
Mem: 1048576 1029360 19216 0 530972 17628
-/+ buffers/cache: 480760 567816
Swap: 1048568 68 1048500
Also, stopping all deamons and dropping cache doesn't help (sum of the
memory used by all processes, displayed by "ps", is about 60 MB):
# echo 1 > /proc/sys/vm/drop_caches
# free
total used free shared buffers cached
Mem: 1048576 453932 594644 0 352 6408
-/+ buffers/cache: 447172 601404
Swap: 1048568 68 1048500
A single "umount" command releases almost 400 MB of memory:
# umount /mnt/iscsi_backup/
# free
total used free shared buffers cached
Mem: 1048576 64188 984388 0 232 6528
-/+ buffers/cache: 57428 991148
Swap: 1048568 64 1048504
What used almost 400 MB? SCSI buffers? I can't find any documentation on
that.
I noticed that when I add RAM to the system, more "unexplained" RAM will
be used.
When I remove some RAM (i.e. so that the system has 512 MB),
"unexplained" RAM usage will drop - as a rule of thumb, I observed that
about 50% of RAM is used by something I can't identify.
--
Tomasz Chmielewski
http://wpkg.org
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