write-intent bitmaps

[Russell Coker]

On Tuesday 29 January 2008 05:15, Bill Davidsen <davidsen at tmr.com> wrote:
> You may have missed the "much higher" part of the previous paragraph.
> And given the reliability of modern drives, unless you have a LOT of
> them you may be looking at years of degraded performance to save a few
> hours of slow performance after a power fail or similar. In other words,
> it's not as black and white as it seems.

What is the pathological case?  1/2 or 1/3 write performance?

For serious write performance of a RAID you want a NVRAM write-back cache for 
RAID-5 stripes, and the NVRAM cache removes the need for write-intent 
bitmaps.  AFAIK Linux software RAID doesn't support such things and that 
putting filesystem journals and the write-intent bitmap blocks on NVRAM 
devices is the best that you could achieve.

It seems that if you want the best performance for small synchronous writes 
(EG a mail server - which may be the most pessimal application for 
write-intent bitmaps) then hardware RAID is the only option.

Are there plans for supporting a NVRAM write-back cache with Linux software 
RAID?

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