I know the RAID5 is one of the most popular raid types in the enterprise market and I saw it in lots of configurations, so I also selected it for my own small array.
One day I read one viewpoint that you would get 56% failure chance when one of the 1T harddisks failed in a RAID5 SATA array and you added a new harddisk to rebuild it.SCSI和SATA的稳定性
I got shocked at that time so when I extended my array I always kept a little nervous until it was finished.
These days I review this point again and I want to figure out where the number 56% comes and I still could not believe the storage or NAS vendors would recommend such array type if they know such rebuild would almost fail certainly.
I found these topics:Why RAID 5 stops working in 2009? Are Fibre Channel and SCSI Drives More Reliable?
And the latter one I guess is the original source of the number 56%. Maybe you could not find it now as the author has corrected it to 44% while you will find it in one of the comments.
So this number is based on below things:
1. Bit Error Rate is 1/10E14
2. The RAID5 array has 8x1TB harddisks
It seems I must be failed as I have a RAID5 array with 7x3TB harddisks in the past, but I should be much safe now as I added one more hardisk and already converted it to RAID6.
But why I could also find many many RAID5 array in enterprise market?
1. Bit Error Rate of enterprise harddisk usually is 1/10E15, which means you will get a bit error when you read about 10E15 bits (10E15/8 is about 125T), while for SATA harddisk this size is just about 12.5T.
2. SMART and some useful functions like Surface Scan of the RAID card or storage controller will remap such bits much early that they really become bad sectors.
One decent discussion could be found here:Statistics on real-world Unrecoverable Read Error rate numbers (not the lies told by vendors on their spec sheets)
So let me back to the subject: Is the RAID5 array right choice for home use?
The answer of mine is : not a good choice now as we already got 10TB harddisk.
RAID6 is much better.
If you have to use RAID5 array, then below guidelines could be helpful:
- Use enterprise hardisks if possible
- Use different batches of the harddisk or different vendors
- Replace your harddisks regularly before they really become failed disks
- ZFS is better alternative solution for RAID5
- The last and the most important one: keep your critical data at least two copies in different locations, like one on local drives and one in the cloud.