Wear Leveling, RAID Can Wipe Out SSD Advantage

storagedude writes "This article discusses using solid state disks in enterprise storage networks. A couple of problems noted by the author: wear leveling can eat up most of a drive's bandwidth and make write performance no faster than a hard drive, and using SSDs with RAID controllers brings up its own set of problems. 'Even the highest-performance RAID controllers today cannot support the IOPS of just three of the fastest SSDs. I am not talking about a disk tray; I am talking about the whole RAID controller. If you want full performance of expensive SSDs, you need to take your $50,000 or $100,000 RAID controller and not overpopulate it with too many drives. In fact, most vendors today have between 16 and 60 drives in a disk tray and you cannot even populate a whole tray. Add to this that some RAID vendor's disk trays are only designed for the performance of disk drives and you might find that you need a disk tray per SSD drive at a huge cost.'"

Comments

Correction:

[raving griff]

Wear Leveling, RAID Can Wipe Out SSD Advantage for enterprise.

While it may not be efficient to slap together a platter of 16 SSDs, it is worthwhile to upgrade personal computers to use an SSD.

Seek time

[1s44c]

The real advantage of solid state storage is seek time, not read/write times. They don't beat conventional drives by much at sustained IO. Maybe this will change in the future. RAID just isn't meant for SSD devices. RAID is a fix for the unreliable nature of magnetic disks.

Re:Raid controllers obsolete?

[TheRaven64]

The advantage of hardware RAID, at least with RAID 5, is the battery backup. When you write a RAID stripe, you need to write the whole thing atomically. If the writes work on some drives and fail on others, you can't recover the stripe. The checksum will fail, and you'll know that the stripe is damaged, but you won't know what it should be. With a decent RAID controller, the entire write cache will be battery backed, so if the power goes out you just replay the stuff that's still in RAM when the array comes back online. With software RAID, you'd just lose the last few writes, (potentially) leaving your filesystem in an inconsistent state.

This is not a problem with ZFS, because it handles transactions at a lower layer so you either complete a transaction or lose the transaction, the disk is never in an inconsistent state.