lkml.org 
[lkml]   [2008]   [Mar]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: RAID-1 performance under 2.4 and 2.6
Bill Davidsen wrote:
> Chris Snook wrote:
>> Bill Davidsen wrote:
>>> Chris Snook wrote:
>>>> Emmanuel Florac wrote:
>>>>> I post there because I couldn't find any information about this
>>>>> elsewhere : on the same hardware ( Athlon X2 3500+, 512MB RAM,
>>>>> 2x400 GB
>>>>> Hitachi SATA2 hard drives ) the 2.4 Linux software RAID-1 (tested
>>>>> 2.4.32
>>>>> and 2.4.36.2, slightly patched to recognize the hardware :p) is way
>>>>> faster than 2.6 ( tested 2.6.17.13, 2.6.18.8, 2.6.22.16, 2.6.24.3)
>>>>> especially for writes. I actually made the test on several different
>>>>> machines (same hard drives though) and it remained consistent across
>>>>> the board, with /mountpoint a software RAID-1.
>>>>> Actually checking disk activity with iostat or vmstat shows clearly a
>>>>> cache effect much more pronounced on 2.4 (i.e. writing goes on much
>>>>> longer in the background) but it doesn't really account for the
>>>>> difference. I've also tested it thru NFS from another machine (Giga
>>>>> ethernet network):
>>>>>
>>>>> dd if=/dev/zero of=/mountpoint/testfile bs=1M count=1024
>>>>>
>>>>> kernel 2.4 2.6 2.4 thru NFS 2.6 thru NFS
>>>>>
>>>>> write 90 MB/s 65 MB/s 70 MB/s 45 MB/s
>>>>> read 90 MB/s 80 MB/s 75 MB/s 65 MB/s
>>>>>
>>>>> Duh. That's terrible. Does it mean I should stick to (heavily
>>>>> patched...) 2.4 for my file servers or... ? :)
>>>>>
>>>>
>>>> It means you shouldn't use dd as a benchmark.
>>>>
>>> What do you use as a benchmark for writing large sequential files or
>>> reading them, and why is it better than dd at modeling programs which
>>> read or write in a similar fashion?
>>>
>>> Media programs often do data access in just this fashion,
>>> multi-channel video capture, streaming video servers, and similar.
>>>
>>
>> dd uses unaligned stack-allocated buffers, and defaults to block sized
>> I/O. To call this inefficient is a gross understatement. Modern
>> applications which care about streaming I/O performance use large,
>> aligned buffers which allow the kernel to efficiently optimize things,
>> or they use direct I/O to do it themselves, or they make use of system
>> calls like fadvise, madvise, splice, etc. that inform the kernel how
>> they intend to use the data or pass the work off to the kernel
>> completely. dd is designed to be incredibly lightweight, so it works
>> very well on a box with a 16 MHz CPU. It was *not* designed to take
>> advantage of the resources modern systems have available to enable
>> scalability.
>>
> dd has been capable of doing direct io for years, so I assume it can
> emulate that behavior if it is appropriate to do so, and the buffer size
> can be set as needed. I'm less sure that large buffers are allocated on
> the stack, but often the behavior of the application models is the small
> buffered writes dd would do by default.
>> I suggest an application-oriented benchmark that resembles the
>> application you'll actually be using.
>
> And this is what I was saying earlier, there is a trend to blame the
> benchmark when in fact the same benchmark runs well on 2.4. Rather than
> replacing the application or benchmark, perhaps the *regression* could
> be fixed in the kernel. With all the mods and queued i/o and everything,
> the performance is still going down.
>

2.6 has been designed to scale, and scale it does. The cost is added
overhead for naively designed applications, which dd is quite
intentionally. Simply enabling direct I/O in dd accomplishes nothing if
the I/O patterns you're instructing it to perform are not optimized. If
I/O performance is important to you, you really need to optimize your
application or tune your kernel for I/O performance.

If you have a performance-critical application that is designed in a
manner such that a naive dd invocation is an accurate benchmark for it,
you should file a bug with the developer of that application.

I've long since lost count of the number of times that I've seen
optimizing for dd absolutely killed real application performance.

-- Chris


\
 
 \ /
  Last update: 2008-03-26 17:45    [W:0.095 / U:0.080 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site