Re: SATA-performance: Linux vs. FreeBSD

[Andi Kleen]

"Martin A. Fink" <fink@xxxxxxxxxx> writes:
> 
> What I did:
> I wrote blocks of 1 MB size to file. Each 1 GB I made a fsync and took the 
> time. For those tests with filesystems I wrote files of 1 GB size, otherwise 
> I just wrote to the raw device.

Newer Linux versions depending on the disk and the file system will tell
the disk to flush the buffers to disk on fsync. FreeBSD might or might not
do that, but if it doesn't it would explain the difference.

> 
> Results: -1-
> 
> Test					OpenSuSE(AHCI)			FreeBSD(AHCI)
> ---------------------------------------------------------------------------------------------------------------------------------------
> SSD(vfat 25GB)			41+/-2 MB/s at 4-10%		15+/-0 MB/s at 2% CPU

vfat is certainly not a performance optimized file system.

> SSD(raw  25GB) 		26+/-1 MB/s at 4-10% CPU	48+/-0 MB/s at 1% CPU
> SSD(ext3 25GB)		39+/-5 MB/s at 10-15% CPU	34+/-0 MB/s at 14% CPU
> SSD(ext2 25GB)		42+/-1 MB/s at 10-15% CPU	32+/-0 MB/s at 10% CPU


You could use oprofile (http://oprofile.sourceforge.net) to find out
where the CPU is being used.


> ---------------------------------------------------------------------------------------------------------------------------------------
> 
> Test					OpenSuSE (AHCI off)		FreeBSD (AHCI off)
> ---------------------------------------------------------------------------------------------------------------------------------------
> SSD(vfat 25GB)			22+/-4 MB/s at 6-19% CPU	--
> SSD(raw  25GB)		33+/-4 MB/s at 7-14% CPU	41+/-0 MB/s at 1% CPU

I remember vaguely (but I might be wrong here) the standard block
character devices on FreeBSD are buffered, while raw is truly
unbuffered on Linux. Naive programs (no optimized IO threads or aio) 
on truly unbuffered devices tend to perform poorly because they
don't do any write behind.

It might also useful if you post the libata related parts of your
boot log.
> 
> Question 2:
> Can anybody explain to me, why writing to a solid state disk (a kind of memory 
> that always has the same constant bandwidth) has such big standard errors in 
> writing rate using Linux (between 1 to 6 MB/s error) while FreeBSD gives an 
> almost constant writing rate (as one would expect it for a SSD) ?

Could be buffered vs unbuffered. Unbuffered single threaded writes
tend to be quite variable.

> Question 3:
> Why is writing to a raw device in Linux slower than using e.g. ext2 ? And why 
> is Linux writing rate much lower (-12.5 % for the best case) compared to 
> writing rate of FreeBSD?

It's really hard to make raw io perform well without complicated
efforts because nobody will hide the IO latencies. That is why
buffered IO is normally recommend

-Andi
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