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.
Since there are already 1 TB SSDs on the market, the only thing
keeping the
HDD market alive is the low $/TB. Moore's Law predicts that cost
advantage
will pass. SSDs are already the low $/IOPS winners.
-- richard
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