Enlarge / I get anxious if I can't watch the blinkenlights in the big Terminal window in the background while the tests run. (credit: Jim Salter)
In earlier coverage pitting ZFS against Linux kernel RAID, some readers had some concerns that we had missed some tricks for mdraid tuning. In particular, Louwrentius wanted us to retest mdadm with bitmaps disabled, and targetnovember thought that perhaps XFS might outperform ext4.
Write intent bitmaps are an mdraid feature that allows disks that have dropped off and re-entered the array to resync rather than rebuild from scratch. The "age" of the bitmap on the returning disk is used to determine what data has been written in its absence—which allows it to be updated with the new data only, rather than rebuilt from scratch.
XFS and ext4 are simply two different filesystems. Ext4 is the default root filesystem on most distributions, and XFS is an enterprise heavy-hitter most commonly seen in arrays in the hundreds or even thousands of tebibytes. We tested both this time, with bitmap support disabled.
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