I plan to do a server with 2 SSDs in RAID1 (for OS and boot) and 3 HDDs in RAID5 (for data). All Linux (CentOS7) with mdadm.
We have in the office 2 leftover SSDs, but they are from different manufacturers and with different capacities (120G and 250G).
I have been reading about RAID1 on SSDs lately and my worries are:
When I create the raid, mdadm will insist on syncing it.
- Both SSDs have been used before. What exactly will mdadm sync when the cells of the two SSDs contain different data anyway?
- Why is that syncing even necessary?
- Does it shorten SSD's life (writing on all cells)?
The
fstrim
command will be issued by a weekly cron job on/
, which will be the mount point of/dev/md0
, which will be the RAID1. Recent versions of mdadm will pass through the trim to the individual hard disks themselves. However, I have read that different SSDs may have different strategies when trimming. For example, one SSD may immediately fill the cells with zeroes, while the other may not. That would mean that the two disks are now out of sync. Now, mdadm has a cron job that, by default, runs periodically and checks the consistency of RAID1 arrays. Will that check report failure because of different trim behaviour?