Monday, 27 June 2011

Fragmentation on ext filesystems

Just had to increase the hard limit for a number of fragments per file on ext filesystem in our ReclaiMe data recovery software, from 64K to 256K. On NTFS, it is not uncommon to see 20,000 or so fragments per file, but ext just beats the hell out of NTFS as far as fragmentation is concerned.

Thursday, 23 June 2011

Difference between RAID 1+0 and RAID 0+1

RAID 10, RAID 1+0, and RAID 0+1 are all the same thing, except for imlpementation details.
All of these have the same data on disks; you cannot tell RAID 1+0 from RAID 0+1 if you just have the disks. Also, performance considerations and fault tolerance are the same for proper implementations of all of these, regardless of what Wikipedia says.

Monday, 13 June 2011

What happens if

What happens if you swap two disks in RAID array while the system is powered down?
If there were, say, three disks A, B, C, on three channels 1, 2, and 3 respectively.
Change the configuration so that disk A is now on channel 2 and disk B is now on channel 1, then power on.

Most likely, nothing happens.

The outcome depends on the method the controller uses to identify the disks. Most if not all modern controllers identify their disks by writing some identification data onto the disks. This way, the controller can tell what the order of disks is by looking at the disks themselves. In earlier days, there were some controllers identifying the disks by ports the disks are connected to. These would be fooled by the swap. Modern controllers (in most cases) carry on just fine.

I just tested it with Silicon Image Sil 3114, and Promise SureTrak EX4650, just in case.
Also, all modern software RAIDs (Linux md-raid and Windows LDM/Dynamic) will also handle this just fine.

Sunday, 5 June 2011

Promise and RAID6

Promise RAID6 array, 4x WD EARS (Green) drives, NTFS-formatted. Copying files to the array maxes out at 2 MB/sec. Quite painful as we already lost about a week trying to create a meaningful test sample.

For reference, controller is Promise SuperTrak EX4650, flashed to the latest firmware.