Writing contigiously to UFS2?

Fluffles etc at fluffles.net
Fri Sep 21 04:09:04 PDT 2007

Ivan Voras wrote:
 > There 4 drives are used in what RAID form? If it's RAID0/stripe, you 
can't avoid data being spread across the drives (since this is the point 
of having RAID0).

It's an array of 8 drives in gconcat, so they are using the JBOD / 
spanning / concatenating scheme, which does not have a RAID designation 
but rather is a bunch of disks glued to each other. Thus, there is no 
striping involved. Offset 0 to 500GB will 'land' on disk0 and then disk1 
takes over, in scheme:

offset 0 
offset 4TB
disk0 -> disk1 -> disk2 -> disk3 -> disk4 -> disk5 -> disk6 -> disk7

(for everyone not familiar with concatenation)

 > If the drives are simply concatenated, then there might be weird 
behavior in choosing what cylinder groups to allocate for files. UFS 
forces big files to be spread across cylinder groups so that no large 
file fills entire cgs.

Exactly! And this is my problem. I do not like this behavior for various 
- it causes lower sequential transfer speed because the disks have to 
seek regularly
- UFS causes 2 reads per second when writing sequentially, probably some 
meta-data thing but i don't like it either
- files are not written contiguously which causes fragmentation, 
essentially UFS forces big files to become fragmented this way.

Even worse: data is being stored at weird locations, so that my energy 
efficient NAS project becomes crippled. Even with the first 400GB of 
data, it's storing that on the first 4 disks in my concat configuration, 
so that when opening folders i have to wait 10 seconds before the disk 
is spinned up. For regular operation, multiple disk have to be spinned 
up which is not practical and unnecessary. Is there any way to force UFS 
to write contiguously? Else i think i should try linux with some linux 
filesystem (XFS, Reiser, JFS) in the hope they do not suffer from this 

In the past when testing geom_raid5 I've tried to tune newfs parameters 
so that it would write contiguously but still there were regular 2-phase 
writes which mean data was not written contiguously. I really dislike 
this behavior.

- Veronica

More information about the freebsd-fs mailing list