I have a zpool consisting of 7 2TB HDDs of different vintage in a raidz setup. Currently, there is neither ZIL nor L2ARC device configured. The server has 12GB of RAM and no swap.
The different filesystems on the pool include /var/spool/imap and /var/db/pgsql. The users aren't many -- just family-members, but sometimes the usage can be heavy, such as when the anti-spam database is retrained (reading the "spam" folders under Cyrus-IMAP and feeding the PostgreSQL DB), or when a free-text search runs through all of the IMAP-messages.
I got a good deal on a new Intel Quantum 32GB device, and am wondering, how to best use it. One obvious thing is to add a Separate Intent Log (SLOG) device. But 32GB seems too much for an infrequently-used pool of 12TB.
The general opinion seems to be, I don't have enough RAM for a meaningful L2ARC. The current ARC-stats are:
ARC: 1680M Total, 441M MFU, 1113M MRU, 32K Anon, 31M Header, 95M Other
1125M Compressed, 1942M Uncompressed, 1.73:1 Ratio
Should I split my new device into a smaller ZIL (4GB?), and use the rest for, say, Cyrus' indices and ccache?
The ZIL SLOG device is intended for Synchronous write activity. This is used by some databases and also services like NFS.
If this is a server where you're running everything on a local ZFS filesystem, writes are going to RAM then flushed to disk. An SLOG isn't really useful for local storage.