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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?

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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.

  • Well, the PostgreSQL db is on the zpool... And so are the Cyrus IMAP-server's files, some of which may be written synchronously... – Mikhail T. Aug 3 '18 at 18:51
  • I doubt it. I don't think an SLOG device will help your situation. – ewwhite Aug 4 '18 at 12:32

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