xfs and Linux Linux 2.6.22 and Memory

Hi All,

I have compiled xfs that is available with Linux 2.6.22 on a Debian 4.0
The system is a 2 cpus (Intel Xeon 3 GHz) with 4 cores each
The system has 8 GB of RAM + 2GB of swap

I have 4 Hard Drives on the system and each is formatted using xfs
Each filesystem is mounted with noatime option I am including the output generated during one of the format:

mkfs.xfs -f /dev/sdb1

meta-data=/dev/sdb1 isize=256
agcount=16, agsize=4881374 blks
= sectsz=512 attr=0
data = bsize=4096
blocks=78101984, imaxpct=25
= sunit=0 swidth=0
blks, unwritten=1

naming   =version 2              bsize=4096
log      =internal log           bsize=4096  

blocks=32768, version=1
= sectsz=512 sunit=0
blks
realtime =none extsz=65536
blocks=0, rtextents=0

I have been using an sql server to populate the 4 xfs filesystems.
So all the files represents part of a relational database.

In each filesystem I have 5 top directory and each directory contains 100,000 files, so in total each filesystem has 500,000 regular files + 5 regular directories.
The sql server is the only program that writes the regular files into the xfs filesystems.

During the time taken to populate the 4 filesystems (2,000,000 files)
I have noticed that approximately 5 GB of RAM were taken.

Once all the filesystems were populated I have shutdown the sql server (the only process that accesses/reads/writes the filesystems) and the 5 GB of RAM would not be released.

At that point I have unmounted the 4 xfs filesystems and the 5 GB of RAM would be released (be available again).

QUESTION:

Is there any way to release that amount of memory without unmounting the file systems ?
Is that caused to some caching mechanism ? Or could that be caused by something else ?

Could you please help ?
Thanks a lot in advance for your help,

Mariella

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It's most likely the linux VFS caching the dentries & inodes, and therefore caching the xfs inodes as well.

cat /proc/sys/fs/dentry-state and /proc/sys/fs/inode-state to see how many are in memory, how many are active, and the age limit.

slabtop would fairly easily show you how much is in the dentry_cache & the inode_cache too.

Are you actually seeing a problem with this (caching is generally good) or just curious?

-Eric

Not just inodes, but file data in the page cache as well.

If you want to blow it all away without unmountin filesystems, then you should read up on /proc/sys/vm sysctls (i.e. /proc/sys/vm/drop_caches)....

Cheers,

Dave.

Dave Chinner
Principal Engineer
SGI Australian Software Group

I was mostly curious, especially because In some cases after having created all the files I may not need to have all of them cached and I may need to availability of the RAM for other purposes.

Thank you all for all the info,

Mariella

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I may be wrong, but as I understand it, if the RAM is needed for other purposes, cache pages are made available automatically and the size of the cache is reduced as the RAM is allocated by processes.