On Fri, 8 Aug 2008, Henrik wrote:
But random writes should be faster on a RAID10 as it doesn't need to
calculate parity. That is why people suggest RAID 10 for datases, correct?
I can understand that RAID5 can be faster with sequential writes.
the key word here is "can" be faster, it depends on the exact
implementation, stripe size, OS caching, etc.
the ideal situation would be that the OS would flush exactly one stripe of
data at a time (aligned with the array) and no reads would need to be
done, mearly calculate the parity info for the new data and write it all.
the worst case is when the write size is small in relation to the stripe
size and crosses the stripe boundry. In that case the system needs to read
data from multiple stripes to calculate the new parity and write the
data and parity data.
I don't know any systems (software or hardware) that meet the ideal
situation today.
when comparing software and hardware raid, one other thing to remember is
that CPU and I/O bandwidth that's used for software raid is not available
to do other things.
so a system that benchmarks much faster with software raid could end up
being significantly slower in practice if it needs that CPU and I/O
bandwidth for other purposes.
examples could be needing the CPU/memory capacity to search through
amounts of RAM once the data is retrieved from disk, or finding that you
have enough network I/O that it combines with your disk I/O to saturate
your system busses.
David Lang
//Henke
8 aug 2008 kl. 16.53 skrev Luke Lonergan:
Your expected write speed on a 4 drive RAID10 is two drives worth, probably
160 MB/s, depending on the generation of drives.
The expect write speed for a 6 drive RAID5 is 5 drives worth, or about 400
MB/s, sans the RAID5 parity overhead.
- Luke
----- Original Message -----
From: pgsql-performance-owner@xxxxxxxxxxxxxx
<pgsql-performance-owner@xxxxxxxxxxxxxx>
To: pgsql-performance@xxxxxxxxxxxxxx <pgsql-performance@xxxxxxxxxxxxxx>
Sent: Fri Aug 08 10:23:55 2008
Subject: [PERFORM] Filesystem benchmarking for pg 8.3.3 server
Hello list,
I have a server with a direct attached storage containing 4 15k SAS
drives and 6 standard SATA drives.
The server is a quad core xeon with 16GB ram.
Both server and DAS has dual PERC/6E raid controllers with 512 MB BBU
There is 2 raid set configured.
One RAID 10 containing 4 SAS disks
One RAID 5 containing 6 SATA disks
There is one partition per RAID set with ext2 filesystem.
I ran the following iozone test which I stole from Joshua Drake's test
at
http://www.commandprompt.com/blogs/joshua_drake/2008/04/is_that_performance_i_smell_ext2_vs_ext3_on_50_spindles_testing_for_postgresql/
I ran this test against the RAID 5 SATA partition
#iozone -e -i0 -i1 -i2 -i8 -t1 -s 1000m -r 8k -+u
With these random write results
Children see throughput for 1 random writers = 168647.33 KB/sec
Parent sees throughput for 1 random writers = 168413.61 KB/sec
Min throughput per process = 168647.33 KB/sec
Max throughput per process = 168647.33 KB/sec
Avg throughput per process = 168647.33 KB/sec
Min xfer = 1024000.00 KB
CPU utilization: Wall time 6.072 CPU time 0.540 CPU
utilization 8.89 %
Almost 170 MB/sek. Not bad for 6 standard SATA drives.
Then I ran the same thing against the RAID 10 SAS partition
Children see throughput for 1 random writers = 68816.25 KB/sec
Parent sees throughput for 1 random writers = 68767.90 KB/sec
Min throughput per process = 68816.25 KB/sec
Max throughput per process = 68816.25 KB/sec
Avg throughput per process = 68816.25 KB/sec
Min xfer = 1024000.00 KB
CPU utilization: Wall time 14.880 CPU time 0.520 CPU
utilization 3.49 %
What only 70 MB/sek?
Is it possible that the 2 more spindles for the SATA drives makes that
partition soooo much faster? Even though the disks and the RAID
configuration should be slower?
It feels like there is something fishy going on. Maybe the RAID 10
implementation on the PERC/6e is crap?
Any pointers, suggestion, ideas?
I'm going to change the RAID 10 to a RAID 5 and test again and see
what happens.
Cheers,
Henke
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