Dave Chinner wrote:
For /some/ RAID controllers in /some/ modes. For example, the
megaraid driver that has been ignoring cache flushes for over 9
years because in RAID mode it doesn't need it. However, in JBOD
mode, that same controller requires cache flushes to be sent because
it turns off sane cache management behaviour in JBOD mode...
---
Lovely. For better or worse, none of my HW-based LSI-raid cards have
been able to do JBOD.
do JBOD.
This is a clear example of why "barriers" should always be on and
cache flushes always passed through to the storage - because we just
don't know WTF the storage is actually doing with it's caches.
----
When it comes to JBOD, its not so clear about where caching
helps the most.
Related -- wondering about how external journals would
affect need for barriers. Haven't thought about it much, but it seems
like one would get alot of bang-for-the-buck to put journals on
SSD's. _If_ data written to SSD's becomes "safe" as soon as it is
accepted by SSDs (not saying it *is*, but _if_ it is), then how
"needed" is ordering of writes for rotating media -- apart from
special use/need cases like apps maintaining their own data-integrity
journals like DB's or such? Just wondering about how increasing use
of SSD's might affect the need for barriers.
Anyway, I have the illusion that I am mostly clear about
current params (at least until my next disillusioning)... ;-)
-l
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