Re: File system performance, hardware performance, ext3, 3ware RAID1, etc.

[Willy Tarreau <willy@w.ods.org>]

On Thu, Feb 12, 2004 at 06:32:31PM -0500, Timothy Miller wrote:
 
> For writes, iozone found an upper bound of about 10megs/sec, which is 
> abysmal.  Typically, I'd expect writes to be faster (on a single drive) 
> than reads, because once the write is sent, you can forget about it. 
> You don't have to wait around for something to come back, and that 
> latency for reads can hurt performance.  The OS can also buffer writes 
> and reorder them in order to improve efficiency.

It depends on the disk too. Lots of disks (specially IDE) are far slower
on writes than they are on reads.

> The 3ware has this write cache that you can turn on or off.  With it 
> off, it ensures that writes make it to the disks in order.  With it on, 
> it will reorder writes more efficiently.  However, I noticed that the 
> performance only went up to about 16meg/sec with the cache ON.

I don't think that the FS type has much importance once the cache is ON.

> IMPORTANT QUESTION:  Is there any metadata anywhere in the swap 
> partition (when it's not in use) that I need to save before I fill it 
> with zeros?

No, there's nothing to save, but you'll have to rebuild the swap at the
end of your tests : mkswap /dev/sga3
I too often use the swap partition for testing purpose or for temporary
storage ; it's easy to do a swapoff; mke2fs on it :-)

> Also, what do I use as a source for zeros when writing with dd? 
> "/dev/zero"?

yes. but don't try to copy any file from the disk onto this partition,
because head seeks will slow things down.

> What's the command?  How about this:
>     time dd if=/dev/zero of=/dev/sga3 bs=1024 count=1024

do it like this, but use higher values, particularly for bs which is only
1kB here. Using something like bs=65536 and count=4096 will give you a 256 MB
file.

> Will that do it?  Should I use an offset to avoid any kind of header or 
> metadata?

not needed. Just ensure that you write to the right partition, and better
check twice.

Regards
Willy