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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.'"
This assumes that RAID controller manufacturers won't be making any changes though.
RAID for years has relied on millisecond access times. So why spend a lot of money on an ASIC & Subsystem that can go faster? So taking a RAID card designed for slow (relatively) spinning disks and attaching them to SSD of course the RAID card is going to be a bottleneck.
However subsystems are going to be designed to work with SSD that has much higher access times. When that happens, this so called 'bottleneck' is gone. You know every major disk subsystem vendor is working on these. Sounds like a disk vendor is sponsoring 'studies' to convince people not to invest in SSD technologies now knowing that a lot of companies are looking at big purchases this year because of the age of equipment after the downturn.
As a rock-in-roll Physicist once said, No matter where you go, there you are.
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.
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.
This study seems to have a very bad case of "unconsciously idealizing the status quo and working from there". For instance:
"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."
That sounds pretty dire. And, it does in fact mean that SSDs won't be neat drop-in replacements for some legacy infrastructures. However, step back for a minute: Why did traditional systems have 50k or 100k RAID controllers connected to large numbers of HDDs? Mostly because the IOPs on an HDD, even a 15K RPM monster, sucked horribly. If 3 SSDs can swamp a RAID controller that could handle 60 drives, that is an overwhelmingly good thing. In fact, you might be able to ditch the pricey raid controller entirely, or move to a much smaller one, if 3 SDDs can do the work of 60HDDs.
Now, for systems where bulk storage capacity is the point of the exercise, the ability to hang tray after tray full of disks off the RAID controller is necessary. However, that isn't the place where you would be buying expensive SSDs. Even the SSD vendors aren't even pretending that SSDs can cut it as capacity kings. For systems that are judged by their IOPS, though, the fact that the tradition involved hanging huge numbers (of often mostly empty, reading and writing only to the parts of the platter with the best access times) HDDs off extremely expensive RAID controllers shows that the past sucked, not that SSDs are bad.
For the obligatory car analogy: shortly after the début of the automobile, manufacturers of horse-drawn carriages noted the fatal flaw of the new technology: "With a horse drawn carriage, a single buggy whip will server to keep you moving for months, even years with the right horses. If you try to power your car with buggy whips, though, you could end up burning several buggy whips per mile, at huge expense, just to keep the engine running..."
... researchers have found that putting a Formula One engine into a Mack truck wipes out the advantages of the 19,000 rpm.
Why is it so hot? Where am I going? What am I doing in this handbasket?
And we don't have to use Highlander Rules when considering drive technologies. There's no reason that one has to build a storage array right now out of purely SSD or purely HDD. Sun showed in some of their storage products that by combining a few SSDs with several slower, large capacity HDDs and ZFS, they could satisfy many workloads for a lot less money. (Pretty much the only thing a hybrid storage pool like that can't do is sustain very high IOPS of random reads across a huge pool of data with no read locality at all.)
I hope we see more filesystems support transparent hybrid storage like this...
If you use ZFS with SSDs, it scales very nicely. There isn't a bottleneck at a raid controller. You can slam a pile of controllers into a chassis if you have bandwidth problems because you've bought 100 SSDs - by having the RAID management outside the controller, ZFS can unify the whole lot in one giant high performance array.
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.
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