linux-kernel - very poor ext3 write performance on big filesystems?

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Date:	Mon, 18 Feb 2008 13:57:16 +0100
From:	Tomasz Chmielewski <mangoo@...g.org>
To:	LKML <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: very poor ext3 write performance on big filesystems?

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?

+ dd if=/dev/zero of=testfile bs=64k count=10000
10000+0 records in
10000+0 records out
655360000 bytes (655 MB) copied, 3,12109 seconds, 210 MB/s
+ sync
0.00user 2.14system 4:06.76elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+883minor)pagefaults 0swaps

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda              1,2T  697G  452G  61% /mnt/iscsi_backup

# df -i
Filesystem            Inodes   IUsed   IFree IUse% Mounted on
/dev/sda                154M     20M    134M   13% /mnt/iscsi_backup

-- 
Tomasz Chmielewski
http://wpkg.org

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