On 05/01/2010 20:19, Richard Elling wrote:
On Jan 5, 2010, at 11:30 AM, Robert Milkowski wrote:
On 05/01/2010 18:49, Richard Elling wrote:
On Jan 5, 2010, at 8:49 AM, Robert Milkowski wrote:
The problem is that while RAID-Z is really good for some workloads
it is really bad for others.
Sometimes having L2ARC might effectively mitigate the problem but
for some workloads it won't (due to the huge size of a working
set). In such environments RAID-Z2 offers much worse performance
then similarly configured NetApp (RAID-DP, same number of disk
drives). If ZFS would provide another RAID-5/RAID-6 like protection
but with different characteristics so writing to a pool would be
slower but reading from it would be much faster (comparable to
RAID-DP) some customers would be very happy. Then maybe a new kind
of cache device would be needed to buffer writes to NV storage to
make writes faster (like "HW" arrays have been doing for years).
This still does not address the record checksum. This is only a
problem
for small, random read workloads, which means L2ARC is a good solution.
If L2ARC is a set of HDDs, then you could gain some advantage, but IMHO
HDD and good performance do not belong in the same sentence anymore.
Game over -- SSDs win.
as I wrote - sometimes the working set is so big that L2ARC or not
there is virtually no difference and it is not practical to deploy
L2ARC several TBs in size or bigger. For such workload RAID-DP
behaves much better (many small random reads, not that much writes).
If you are doing small, random reads on dozens of TB of data, then you've
got a much bigger problem on your hands... kinda like counting grains of
sand on the beach during low tide :-). Hopefully, you do not have to
randomly
update that data because your file system isn't COW :-). Fortunately,
most
workloads are not of that size and scope.
Well, nevertheless some environments are like that (and no, I'm not
speculating) and the truth is that NetApp with RAID-DP with the same
amount of disk drives proven to be faster than RAID-Z2 even with a help
of SSDs as L2ARC. The point is that NetApp allowed to provide the
capacity of RAID-6 and protection of dual parity while providing better
performance to RAID-Z2 in the environment.
In other workloads RAIDZ-2 will be better, but not in this particular
environment.
All I'm saying is that having yet another RAID type in ZFS which offers
capacity similar to RAID-5/RAID-6 but with different performance
characteristics so small random reads are on par with RAID-DP while
sacrificing write performance would be beneficial for some environments.
RAID-Z with bigger sector size could improve performance but provided
capacity could be much less than RAID-5/6 so it not necessary might be
an apple-to-apple comparison (but still useful for some environments).
--
Robert Milkowski
http://milek.blogspot.com
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