Thursday, November 12, 2009

vmstat limitations

In the aftermath of the previous post, I ended up monitoring the progress of several huge pvmove and md RAID1 resync operations. Running "vmstat 1" however only shows pvmove block I/O activity, but not md RAID1 resyncing. Fortunately, "dstat -d" shows the reads and writes of both pvmove and md RAID1 resync. ("-d" is automatically included in "dstat -a" by the way.)
N.B. This only affects block I/O measurements, you'll still see the impact on interrupts and CPU statistics in vmstat.

Saturday, November 7, 2009

hotplug SATA with CentOS 5

When the SATA standard was introduced in PCs, I'd read that the electrical connections of both the data and the power connector had been designed with hotplug in mind. But just as with many hotplug-able technologies, I never actually tried it (hotplug PCI, anyone ?). Until now, that is.

Job at hand: replace the software-mirrored SATA drives in high-end Dell workstations with bigger ones. Without losing the data, of course. A perfect opportunity to test how Linux handles SATA hotplug ! Actually, adding the drive was a breeze, Linux automatically detects the drive, and I could sfdisk, and mdadm --add. Next step was hot-removing the old drives: mdadm --fail and mdadm --remove, then physically unplug the drives. I didn't expect it to be so easy, to be honest. Great technology !