[Gregory Leblanc]
> > Recovery is a tad simpler with raid1 done at the lower level simply
> > because none of the md device ever "dies", just one falls
> > into degraded
> > and you can skip an mkraid and let normal recovery take over.
> > Of course,
> > that leaves the raid1 read balancing algorithm (arguably the
> > weak point in
> > the read performance of 0+1 or 1+0) running in two places
> > instead of one.
>
> Could you elaborate a little? Are you talking about the default 0.90 code,
> or patched with Mika's brilliant patch? Theoretically, RAID1+RAID0 should
> be extreemly fast for reads, and only a bit slower for writes, assuming that
> you're not saturating the bus.
Mika's patch is a straightforward one that improves small, random (ie
seek-heavy) reads well. I haven't seen it (in my experience) improve
large sequential reads to the point of raid0 (just in my testing), but
it's an issue Mika and I have hashed over many other times, and it's
not worth banging over again on this list.
Thankfully, it's now a largely moot issue in the cases I need as
madvise(MADV_SEQUENTIAL) is around so I can get async forward page-in's
(the main reason I don't care about seq raid1 read perf much anymore,
and why I added the mmap/madvise code to tiobench)
James
