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09/04/2012 08:45
Filesystems for USB drive Backup

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Glen Pitt-Pladdy :: Blog

Too hot to handle

I use dirvish (a neat Perl wrapper for rsync) with LUKS encryption for my backups.  This does "snapshot" style backups with unchanged files hard linked against the previous version which means that I can very efficiently store many versions of the filesystem, and each is a complete copy of all the files on the system. The encryption is very useful if a drive fails as I don't have to worry about destroying data on an inaccessible device.  It also makes it much easier to store off site - I don't have to worry about my data falling into the wrong hands since it's no use to them as it is.

FSCK!  It happened again!

As I type, once again one of my USB drives is giving trouble. This particular drive has developed a habit of running for a while and then loosing the USB connection. This means that the filesystem is left in an unclean state and then needs to have fsck run on it to clear any problems.

Initially I was running ext2 on it simply to avoid the overhead of a journal over the limited bandwidth of USB, but decided to switch to JFS for the sake of quick recovery (XFS is also an option, but I have had occasional problems in the past and JFS seems to be as good and so far has worked perfectly for me).

Initially I suspected the USB to SATA electronics in the case, so to clear any problems, I removed the drive and put it on another USB to SATA converter.  After some running in free air the drive was absolutely scorching hot - way to hot to touch. When I was designing power electronics we had a simple rule of thumb: if you couldn't touch it, it was probably running too hot for long term reliability.

When idle, the drive seems fine, but dirvish don't half make the drive hot. I suspect the problem is that with so many files being hard linked by dirvish, most of the operation of the drive is seeking, and that seems to be where the heat is coming from.

Living in a box

USB drives are simply standard desktop drives (or laptop drives in the case of "portable" USB drives), in a case with a bit of electronics that translates USB to SATA (or PATA in older ones), sadly without SMART to be able to monitor the condition of the actualy drive. Normally, these drives would be in a case with lots of free air around them and in good quality PC cases, guaranteed airflow from the design of the case and/or fan cooling.

What happens when we take the same device and put it in a cosy little box with no ventilation, no space for air circulation and no fans?

Maybe I just happen to have got a particularly hot running drive, but I can't help wondering if these USB drives without good cooling are operating a little close to the limit when it comes to temperature, and the aggressive seeking with dirvish hard linking thousands of files is something that was never considered when they where designed. Quite happy to run mostly idle, or transfer large files with minimal seeking, but not happy to seek like crazy. Then again.... maybe some weren't "designed" in the sense that anyone paid much attention to good operating margins compared making funky looking boxes.

Fortunately newer drives run much cooler so this problem will probably go away as newer models are released.

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