RAID Lag-Behind Burst Write

Question

I would like to setup RAID1 so that a ramdisk in the RAID configuration has occasional synchronisation with a physical disk (that is very battery intensive to run, so I hope to let it spinout).

Is there a way I can set the commission frequency on the RAID config such that it will only burst-write to the HDD every, let's say, 5 minutes?

Answer 1

If you use mdadm for your software-raid-1 and set it up with a dirty-map, you could connect/disconnect that disk every N seconds to the md-device.

In md-terms that is

• set faulty
• remove
• re-add

Answer 2

Well, for some FSes you can have such tweaks, say for EXT3, EXT4 man mount mentions «…

commit=nrsec Sync all data and metadata every nrsec seconds. The default value is 5 seconds. Zero means default.

…» — That might suffice for your needs.

Also, there's vm.dirty_writeback_centisecs sysctl which is described as: «…

The pdflush writeback daemons will periodically wake up and write `old' data out to disk. This tunable expresses the interval between those wakeups, in 100'ths of a second.

Setting this to zero disables periodic writeback altogether.

…»

So, actually probably you don't have to mess with RAM-disk/HDD RAID1 at all.