Actually for the ZIL you may use the a-card (memory sata disk + bbu + compact
flash write out).
For the data disks there is no solution yet - would be nice. However I prefer
the "supercapacitor on disk" method.
Why ? because the recharge logic is chellenging. There needs to be
communication between the disk and the power supply. The interesting case is
"fluctucating power" (see below) and battery maintenance.
If you are charged you can run fine, but the corner cases are tricky.
Image the following scenario:
1) Operations: Normal
2) Power outage: 1 hour
3) UPS failing after 30 minutes
4) Power comes back
5) ALL Servers power on at the same time (e.g. misconfiguration)
6) Peak -> Power goes down again
At 3) your batteries are empty.
At 6) your batteries are not fully charged, however because the device does not
know the "status" of the local UPS, write cache is still enabled.
Thus a simple design does not solve the problem well (eneugh).
Another thing is maintenance of a battery. You have to check if your battery
still works (charge cycle). You have to alarm if not (monitoring). You have to
replace them online then. So in general - batteries are bad if your server
lifes longer then 3 years :)
For google it works fine, maybe because the server will life < 3 years anyhow
and because they can "jus treplace" the server due to their internal redundancy
options (google backend technology is designed to handle failure well). For a
storage system I don't see that.
The BBU / Capcitor needs to implement the same logic a Raid BBU implements.
If (not_working_fully(BBU)) {
disable_write_cache();
} else {
enable_write_cache();
}
Or better (explict state whitelisting guaranteeing data integrity also for
unexpected states):
If (working_fully(BBU)) {
enable_write_cache();
} else {
disable_write_cache();
}
p.s. While writing this I'm thinking if a-card handles this case well ? ...
maybe not.
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