[nycbug-talk] File Backed Disks- Speed Issues

Isaac Levy ike at lesmuug.org
Thu Sep 28 14:15:23 EDT 2006

Hey All,

So I'm sad, and wanted to solicit some help to perhaps cheer me up.

I've been using File-Backed disks via mdconfig(8) with FreeBSD jail 
(8) for a very long time.
(I tend to call them 'disk images', Apple vocabulary.)

Creating a New File-Backed Disk with mdconfig:

Their usefulness, in my enviornment, is that they are far more  
flexible to work with in constraining disk size for jails across  
machines than any other strategy, (namely, hard disk partitions).

My sadness, and my query has to do with speed- or the painful lack of  
Running a disk image is notably slower than the raw disk, (in my  
case, fairly snappy RAID5/SATA), but 'usable'.
Based on totally unscientific benchmarking, when I've got more than  
twenty disk images on a given machine, (all running jails), disk I/O  
becomes far slower than is usable- and each new disk takes the speed  
down at what seems to be an order of significant magnitude.

What I'm saying is, they don't scale.

During the FreeBSD 4.x days, (wow what an era), file-backed disks  
were snappy, with little overhead, no? (vn(4) and vnconfig(8),  

In the immediate future, I'm taking some jailing systems back to just  
running jails on the filesystem, (and partitioning, yuck), but in the  
meantime does anyone have any thoughts on the subject?  It simply  
seems like the file-backing part of the memory disk implementation  
was slapped on, or perhaps is just cruft left over from the 5.x era?



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