very poor ext3 write performance on big filesystems?
From: Tomasz Chmielewski
Date: Mon Feb 18 2008 - 07:57:36 EST
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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