[CentOS] Centos/Linux Disk Caching, might be OT in some ways

Noob Centos Admin centos.admin at gmail.com
Sun Jan 24 17:09:15 UTC 2010


I'm trying to optimize some database app running on a CentOS server
and wanted to confirm some things about the disk/file caching
mechanism.

>From what I've read, Linux has a Virtual Filesystem layer that sits
between the physical file system and everything else. So no matter
what FS is used, applications are still addressing the VFS. Due to
this, disk caching is done on an inode/block basis.

I'm assuming that this is still the case in CentOS or am I badly mistaken?

If that is correct, then here is my scenario and hypothesis.

Assuming the server has xxx MB of free memory and the database consist
of several tables more than xxx MB in size. So no table will fit
entirely into memory. And assuming other processes do not interfere
with the caching behaviour or available memory etc.

Given the inode caching behaviour, if the DBMS only access a bunch of
inodes that total less than xxx MB, is it therefore likely to be
always using the cache, hence faster?

My thought is that if this is the case, then I could likely speed up
the application behaviour if I further split the tables into parts
that are more frequently accessed, and parts that are unlikely
touched.

e.g. the table may currently have rows with 20 fields and total
1KB/row, but very often say only 5/20 fields are used in actual
processing. Reading x rows from this table may access x inodes which
would not fit into the cache/memory.

However if now I break the table into two parts with those 5 fields
into a smaller table, there would be a speed increase since the
reading the same x rows from this table would only access 1/x inodes.
Further more, these would more likely fit into the disk/memory cache
for even faster access.

Or would I simply be duplicating what the DBMS's index files would
already be doing and therefore see no improvement?


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