does SCSI eat my PC's memory?

does SCSI eat my PC's memory?

[Tomasz Chmielewski]

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 noticed that when I add RAM to the system, more "unexplained" RAM will 
be used.
When I remove some RAM, "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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