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Hybrid Storage Solutions Compared
Vigile writes "While few would argue with the performance advantages of solid state drives, the relative cost compared to spindle-based disks still make them a luxury item. The promise of hybrid storage solutions is to combine the benefits of both — large capacities with standard drive technology and performance advantages of solid state. PC Perspective published an article comparing several different solutions that vary in their approach to hybrid storage. The OCZ RevoDrive Hybrid combines a standard 2.5-in drive with a PCI Express-based SSD that offers the best overall performance and largest cache size. Seagate's new Momentus XT 2.5-in solution embeds the cache on the PCB of the drive, allowing notebook users to install this solution easily. Finally, the Intel chipset-based caching option combines either a 2.5-in or mSATA SSD with a standard hard drive on either desktop or mobile platforms, allowing the most flexibility of any other hybrid solution. All three have advantages for specific consumers, though, and varying performance levels to go along with them."
The limit of an ssd's max writes is high enough that time wise a mechanical hard drive is more likely to fail during the normal lifespan.
Facebook released, some time ago, their Flashcache solution. It works similar to ReadyBoost, et al, except it works on Linux, and "pairs" an SSD with a hard drive. Very useful.
http://www.facebook.com/note.php?note_id=388112370932
Yeah, well, you're wrong: http://maxschireson.com/2011/04/21/debunking-ssd-lifespan-and-random-write-performance-concerns/
Seagate's new Momentus XT 2.5-in solution
Had a 500 GB version in my laptop since they came out last year (Summer I think) And yes, it's much faster than a typical 7200 rpm drive.
The best thing about a boolean is even if you are wrong, you are only off by a bit.
According to Seagate the Momentus XT will fail back to being a regular hard drive if flash failure is detected by the controller. All data in flash is also stored on the drive, the SSD part only caches a copy of already stored data for faster read performance.
EA David Gardner -"... but the consumers have proven that actually what they want is fun."
What I want is a filesystem that can use a partition of an SSD and a partition of a rotating magnetic disk. Metadata, directories and small files on the SSD, big files on the rotating disk.
This ought to be fairly simple - anyone fancy hacking ext4?
According to Seagate the Momentus XT will fail back to being a regular hard drive if flash failure is detected by the controller. All data in flash is also stored on the drive, the SSD part only caches a copy of already stored data for faster read performance.
That's obviously true for reads, but is it true for writes?
These hybrid devices typically have a battery (excuse me, "super capacitor") to flush any cached writes out to disk.
But what if the OS thinks data was written (because it went to the SSD cache successfully), but flushing from cache to disk fails because something broke on the SSD side?
SSD controllers haven't been exactly stellar in terms of reliability so far.
For my money, I just got two 256 GB Crucial M4s, and I do daily full-image backups (excluding *.bt! and any steam games) to an external 2 TB drive.
Doesn't really matter.
Anand from Anandtech writes:
My personal desktop sees about 7GB of writes per day. That can be pretty typical for a power user and a bit high for a mainstream user but it's nothing insane. ...
If I never install another application and just go about my business, my drive has 203.4GB of space to spread out those 7GB of writes per day. That means in roughly 29 days my SSD, if it wear levels perfectly, I will have written to every single available flash block on my drive. Tack on another 7 days if the drive is smart enough to move my static data around to wear level even more properly. So we're at approximately 36 days before I exhaust one out of my ~10,000 write cycles. Multiply that out and it would take 360,000 days of using my machine for all of my NAND to wear out; once again, assuming perfect wear leveling. That's 986 years. Your NAND flash cells will actually lose their charge well before that time comes, in about 10 years.
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Adaptec has a new technology called Hybrid RAID, which uses 50% SSD/50% HDD in RAID 1 or RAID 10. Reads are serviced by the SSD(s) only. It seems to me you might need to short-stroke the HDD in order to make this a reasonable approach.
Their new 6E series is very inexpensive for 6Gb SAS cached (128MB) hardware RAID; 6805E is (8 int. ports) is about $225 retail:
https://www.adaptec.com/en-us/products/controllers/hardware/sas/entry/sas-6805e
I'm no shill or fanboy; I just found this interesting and relevant.
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That analysis is for high-end enterprise-class SSDs that use SLC memory. The lifespan for consumer-class MLC-based SSDs is much worse.
That's obviously true for reads, but is it true for writes?
The Momentus XT only caches reads. Writes (i.e., data from the computer to the drive) completely bypass the cache.
If you can add 4-8GB of RAM, you're better off spending your money on that and a standard 3-1/2" mechanical hard drive. The Momentus XT really is only useful in laptops.
Reading from SSD is insanely faster than reading from SAS. Writing to SSD is much slower. There is no way around that.
Those hybrids products are simply a futile tentative to come up with a cheap alternative to what is really needed: adding a decent cache on SSD controllers so the write buffer is big enough to mitigate the write penalty, and adding enough processing power to perform destaging properly.
This being said - if someone comes up with a way to read from SSD and write to SAS, then it's a winner... but the magic part that brings the written bit from the SAS to be read by the SSD is the million-dollar catch.
Until then, those hybrids are just a Fisher Price implementation of sub-volume tiering.
lucm, indeed.
You can purchase 24 GB of ram for less than $200 today. I would love to see a side by side performance and energy consumption comparison with someone who decided to spend their extra money on DRAM.
I suspect what you'll see is that after a day all read operations are resolved instantly from the OS disk cache without the performance and power hit of flash. If you have write intensive workloads the current crop of SSDs would not be for you anyway.
That article talks about enterprise drives which use SLC flash. SLC has 20 - 30 times the write endurance of the MLC flash you get in consumer-grade SSDs.
SSD controllers are good enough now that I wouldn't worry about the MLC flash in my laptop's SSD for general use, but I'd take a very close look at the numbers if I was using it to do anything that was write heavy (like video work or building a big codebase).
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That analysis is for high-end enterprise-class SSDs that use SLC memory. The lifespan for consumer-class MLC-based SSDs is much worse.
And consumer drives are generally less write heavy as well. Newer enterprise class SSDs are MLC as well. Given the cheapness of MLC, you can increase the amount of FLASH available to make up for the lack of erase cycles, and using deduplication like the SandForce drives, you write less anyway (I wonder how SF drives do GC.)
Simple fact is that firmware errors or user errors will lose your data before the FLASH wears out.
The problem with most hybrid disks is the fact that they don't know about filesystems, they don't know about current working conditions of the rest of the system and they don't know of user and application preferences: they're working with a very limited set of data available to them and thus they can only base their operation on guesses of what is going to be needed next. The OCZ RevoDrive is the exception here in that it's actually two drives that are both accessible separately to the OS and/or applications if one doesn't install their caching software.
OS- and filesystem-agnostic system is always going to be somewhat limited so it would be beneficial if the rest of these hybrid systems also allowed the OS direct access to the flash so that the OS can optimize its behaviour accordingly. Like e.g. the user is reading several multi-gigabyte files, say, disc images for example; a OS- and filesystem-agnostic hybrid will start caching those files since it doesn't know how large those files are or how often they're needed, whereas if OS handled the caching it could determine that those files are accessed very rarely, usually in large sequences, and thus it would be better to skip caching them and instead save the cache for any related smaller files that are accessed more often. If you're familiar with the Windows ReadyBoost it actually works similarly: it skips caching large files as usually mechanical drives are more than fast enough for large sequential reads, but it instead caches a helluva lot of all kinds of small files since flash is absolutely terrific at random accesses and reading in even hundreds of small files from a slow flash is still faster than reading them in from the fastest mechanical one (I have a class10 16GB SD-card in my laptop as ReadyBoost drive and I've certainly noticed huge improvement in application startup times)
So yeah, sure, do create hybrids, they're a good idea in general. Just allow the OS to flip a bit and gain direct access to the flash part, too, for those OSes that are aware of such.
for some reason I thought this article may have been about Prius's or wind/solar power, but.....
There was an unknown error in the submission.
SSD controllers haven't been exactly stellar in terms of reliability so far.
Yeah. This post is quasi a dupe of a few days ago where this eye opening message was posted. ,and I'm really not sure I want an hybrid.
I have converted most of my systems to SSDs for the system disk and now I'm scared. I'm sure there's an OCZ somewhere in there... Add to this the risk of mechanical failure and added complexity
Non-Linux Penguins ?
Putting both in the same can is the wrong way to do it.
A better way is to have SSD and hard disk as tiered storage. The OS is more likely to be able to make better decisions for what to store where. So think in terms of overmounting the SSD onto the HD file system.
The OS on reading a file from the HD looks at the access pattern and if it gets accessed frequently, it writes it out to the SDD. If a file hasn't been accessed for a while it gets deleted from the SSD if it's clean, written to the HD and deleted if it's dirty.
When a file is created it's written to the SSD. If it's not accessed for X hours, it's flushed to HD. If it's not accessed for Y hours (Y>X) it's deleted from HD.
With separate SSD/HD you can pick your components. You can also do things like mirror your SSD and RAID your HDs.
Surprised this sort of FS access not available on Linux/BSD yet.
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Multiply that out and it would take 360,000 days of using my machine for all of my NAND to wear out
The number to be concerned about isn't when all of the cells wear out, it's when one more than you need for cell redundancy wears out.
My God, it's Full of Source!
OUTSIDE_IP=$(dig +short my.ip @outsideip.net)
I just built a system with this motherboard, and while I haven't run any benchmarks, it does seem to be very fast.
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