The issue happened again. I noticed there could be a link with the fact that the computer is up for days and then at next reboot it triggers a reconstruction. (the 3 disks composing the raid device are healthy) The computer is very slow during reconstruction.
sudo dmesg | grep md127: [ 18.394466] md/raid:md127: not clean -- starting background reconstruction [ 18.394475] md/raid:md127: device sdb1 operational as raid disk 1 [ 18.394476] md/raid:md127: device sdc1 operational as raid disk 2 [ 18.394476] md/raid:md127: device sda1 operational as raid disk 0 [ 18.394697] md/raid:md127: raid level 5 active with 3 out of 3 devices, algorithm 2 [ 18.394765] md127: bitmap file is out of date (10910 < 10911) -- forcing full recovery [ 18.394772] md127: bitmap file is out of date, doing full recovery [ 18.479938] md127: detected capacity change from 0 to 1717716385792 [ 18.485209] md127: AHDI p1 p3 [ 18.485234] md127: p1 size 3214817840 extends beyond EOD, truncated [ 29.228966] md127: AHDI p1 p3 [ 29.228970] md127: p1 size 3214817840 extends beyond EOD, truncated [ 29.554661] md: resync of RAID array md127 cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] [linear] [multipath] [raid0] [raid1] [raid10] md127 : active raid5 sdb1[1] sdc1[3] sda1[0] 1677457408 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] [UUU] [================>....] resync = 80.0% (671298352/838728704) finish=22.9min speed=121385K/sec bitmap: 7/7 pages [28KB], 65536KB chunk Eric