Re: very poor ext3 write performance on big filesystems?

[Vladislav Bolkhovitin <vst@vlnb.net>]

Tomasz Chmielewski wrote:
> I have a 1.2 TB (of which 750 GB is used) filesystem which holds
> almost 200 millions of files.
> 1.2 TB doesn't make this filesystem that big, but 200 millions of files 
> is a decent number.
> 
> 
> Most of the files are hardlinked multiple times, some of them are
> hardlinked thousands of times.
> 
> 
> Recently I began removing some of unneeded files (or hardlinks) and to 
> my surprise, it takes longer than I initially expected.
> 
> 
> After cache is emptied (echo 3 > /proc/sys/vm/drop_caches) I can usually 
> remove about 50000-200000 files with moderate performance. I see up to 
> 5000 kB read/write from/to the disk, wa reported by top is usually 20-70%.
> 
> 
> After that, waiting for IO grows to 99%, and disk write speed is down to 
> 50 kB/s - 200 kB/s (fifty - two hundred kilobytes/s).
> 
> 
> Is it normal to expect the write speed go down to only few dozens of 
> kilobytes/s? Is it because of that many seeks? Can it be somehow 
> optimized? The machine has loads of free memory, perhaps it could be 
> uses better?
> 
> 
> Also, writing big files is very slow - it takes more than 4 minutes to 
> write and sync a 655 MB file (so, a little bit more than 1 MB/s) - 
> fragmentation perhaps?

It would be really interesting if you try your workload with XFS. In my 
experience, XFS considerably outperforms ext3 on big (> few hundreds MB) 
disks.

Vlad