I have a RAID 5 on 3 disks 4K aligned (2x WD2001FFSX and 1x WD20EFRX).One of the disks has failed, and i replaced it with an other WD2001FFSX.After partitioning it the same way than the other (1 partition getting the entire space, aligned at 2048 sectors), i add it to the raid.But see by yourself :mick@baloo2:~$ cat /proc/mdstat Personalities : [raid6] [raid5] [raid4] md0 : active raid5 sde1[0] sdb1[3] sdd1[1]      3906763776 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/2] [UU_]      [>....................]  recovery =  0.2% (5345880/1953381888) finish=81762.7min speed=396K/secunused devices: <none>mick@baloo2:~$At this speed, this will get about 2 month to accomplish !Is there anybody here able to point what is wrong in my configuration ?Here are some useful informations :mick@baloo2:~$ sudo mdadm -E /dev/sdb1/dev/sdb1:      Magic : a92b4efc    Version : 1.2    Feature Map : 0x2     Array UUID : 6d208e00:02a3bdac:793d2f47:81af4052       Name : akela:0  Creation Time : Sat Apr 16 23:48:38 2016     Raid Level : raid5   Raid Devices : 3 Avail Dev Size : 3906764976 (1862.89 GiB 2000.26 GB)     Array Size : 3906763776 (3725.78 GiB 4000.53 GB)  Used Dev Size : 3906763776 (1862.89 GiB 2000.26 GB)    Data Offset : 262144 sectors   Super Offset : 8 sectorsRecovery Offset : 2505736 sectors      State : clean    Device UUID : 840de8ae:f4721e56:71cac3dc:7f9aae2f    Update Time : Sun May 29 15:31:02 2016       Checksum : edfdcb1f - correct     Events : 90414     Layout : left-symmetric     Chunk Size : 512K   Device Role : Active device 2   Array State : AAA ('A' == active, '.' == missing)mick@baloo2:~$ mick@baloo2:~$ mick@baloo2:~$ mick@baloo2:~$mick@baloo2:~$ sudo mdadm -E /dev/sdd1/dev/sdd1:      Magic : a92b4efc    Version : 1.2    Feature Map : 0x0     Array UUID : 6d208e00:02a3bdac:793d2f47:81af4052       Name : akela:0  Creation Time : Sat Apr 16 23:48:38 2016     Raid Level : raid5   Raid Devices : 3 Avail Dev Size : 3906764976 (1862.89 GiB 2000.26 GB)     Array Size : 3906763776 (3725.78 GiB 4000.53 GB)  Used Dev Size : 3906763776 (1862.89 GiB 2000.26 GB)    Data Offset : 262144 sectors   Super Offset : 8 sectors      State : clean    Device UUID : 226eefd7:6da9b123:4240d44b:df2c9dd1    Update Time : Sun May 29 15:31:02 2016       Checksum : f5712e5b - correct     Events : 90414     Layout : left-symmetric     Chunk Size : 512K   Device Role : Active device 1   Array State : AAA ('A' == active, '.' == missing)mick@baloo2:~$ mick@baloo2:~$ mick@baloo2:~$ mick@baloo2:~$ sudo mdadm -E /dev/sde1/dev/sde1:      Magic : a92b4efc    Version : 1.2    Feature Map : 0x0     Array UUID : 6d208e00:02a3bdac:793d2f47:81af4052       Name : akela:0  Creation Time : Sat Apr 16 23:48:38 2016     Raid Level : raid5   Raid Devices : 3 Avail Dev Size : 3906764976 (1862.89 GiB 2000.26 GB)     Array Size : 3906763776 (3725.78 GiB 4000.53 GB)  Used Dev Size : 3906763776 (1862.89 GiB 2000.26 GB)    Data Offset : 262144 sectors   Super Offset : 8 sectors      State : clean    Device UUID : 1e381c08:c0a750d5:f85a153b:71d8df29    Update Time : Sun May 29 15:31:02 2016       Checksum : 1ec0bcf1 - correct     Events : 90414     Layout : left-symmetric     Chunk Size : 512K   Device Role : Active device 0   Array State : AAA ('A' == active, '.' == missing)mick@baloo2:~$fdisk -l give me the following :Disk /dev/sdb: 2000.4 GB, 2000398934016 bytes81 heads, 63 sectors/track, 765633 cylinders, total 3907029168 sectorsUnits = sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 4096 bytesI/O size (minimum/optimal): 4096 bytes / 4096 bytesDisk identifier: 0xc97c3544   Device Boot      Start         End      Blocks   Id  System/dev/sdb1            2048  3907029167  1953513560   fd  Linux raid autodetectDisk /dev/sdd: 2000.4 GB, 2000398934016 bytes255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectorsUnits = sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 4096 bytesI/O size (minimum/optimal): 4096 bytes / 4096 bytesDisk identifier: 0x00000000   Device Boot      Start         End      Blocks   Id  System/dev/sdd1            2048  3907029167  1953513560   fd  Linux raid autodetectDisk /dev/sde: 2000.4 GB, 2000398934016 bytes255 heads, 63 sectors/track, 243201 cylinders, total 3907029168 sectorsUnits = sectors of 1 * 512 = 512 bytesSector size (logical/physical): 512 bytes / 4096 bytesI/O size (minimum/optimal): 4096 bytes / 4096 bytesDisk identifier: 0x00000000   Device Boot      Start         End      Blocks   Id  System/dev/sde1            2048  3907029167  1953513560   fd  Linux raid autodetect
What can slow down a RAID 5 with 3 disks recovery rate to sub-MB speeds?
raid;mdadm