[Lustre-discuss] Using SSDs for OSS Read Cache on servers with little RAM?

[Scott Adametz]

Any possibility to redirect Lustre's OSS Cache or portions of the Linux
pagecache to a location other than RAM such as to an internal SSD (perhaps
even two in RAID0).  I ask because the servers we have available (read:
second hand servers) for OSSs physically max out at 8GB RAM.

The wiki states: *"It uses a regular Linux pagecache to store the data."*  I
looked into pagecache and pdflush tunables but found little on directing the
actual location of the cached content as it tends to focus heavily on tuning
timing of writes-*to*-disk vs reads.    Yes, I understand the absurdity of
trying to redirect RAM cached content back to "disk" but with rapidly
evolving SSD design and performance it actually makes some sense in this
case for Lustre cache data.  Obviously I would prefer to redirect
*only*Lustre OSS cache data and not the entire Operating System page
cache.

While an SSD is nowhere near as quick as RAM, it would still seem beneficial
to have more frequently viewed content available for reads even from an SSD
vs a spinning disk, especially with our use case (HD video editing and
playback).  Until SSD prices come down to reasonable levels and we can
afford to replace all spinning disks with SSDs, any little bit helps ;-)

In sum: just a thought to be able to designate, in Lustre configs, an
additional low-latency "swap" or "cache" location on the OSSs.  Adding one
SSD to each OSS sure beats having to buy new servers with more RAM capacity
or using all SSD drives and blowing the budget...

Thanks for the time,

Scott Adametz
Systems Engineer
Big Ten Network
FOX Networks Group
scott.adametz at fox.com
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