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SubjectRe: EXT4 is ~2X as slow as XFS (593MB/s vs 304MB/s) for writes?
Eric Sandeen wrote:
> Justin Piszcz wrote:
>> Hello,
>>
>> Is it possible to 'optimize' ext4 so it is as fast as XFS for writes?
>> I see about half the performance as XFS for sequential writes.
>>
>> I have checked the doc and tried several options, a few of which are shown
>> below (I have also tried the commit/journal_async/etc options but none
>> of them get the write speeds anywhere near XFS)?
>>
>> Sure 'dd' is not a real benchmark, etc, etc, but with 10Gbps between 2
>> hosts I get 550MiB/s+ on reads from EXT4 but only 100-200MiB/s write.
>>
>> When it was XFS I used to get 400-600MiB/s for writes for the same RAID
>> volume.
>>
>> How do I 'speed' up ext4? Is it possible?
>>
>
> FWIW I'm seeing similar things on fast storage (Fusion IO),
> though this is under 2.6.31. 500MB/s+ for xfs, 300 for ext4.
>
> Overwriting an existing file is no faster. I don't think this
> driver is blktraceable but I'll try a newer driver that should be I think.

FWIW, blktrace (I'm still on 2.6.31) is enlightening:

Total (xfs):
Reads Queued: 4, 16KiB Writes Queued: 122,567, 10,485MiB
Read Dispatches: 4, 16KiB Write Dispatches: 83,219, 10,485MiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 4, 16KiB Writes Completed: 83,219, 10,485MiB
Read Merges: 0, 0KiB Write Merges: 39,348, 314,804KiB
IO unplugs: 344 Timer unplugs: 338

Total (ext4):
Reads Queued: 14, 56KiB Writes Queued: 2,621K, 10,486MiB
Read Dispatches: 14, 56KiB Write Dispatches: 107,944, 10,486MiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 14, 56KiB Writes Completed: 107,944, 10,486MiB
Read Merges: 0, 0KiB Write Merges: 2,513K, 10,054MiB
IO unplugs: 2,461 Timer unplugs: 2,020


See "Writes Queued" See also submit_bio() calls in xfs.

ext4 doing things a block at a time is certainly giving the elevator a workout...
I'd tend to chalk it up to that at first glance.

-Eric


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