Slashdot Mirror
Is the Time Finally Right For Hybrid Hard Drives?
a_hanso writes "Hard drives that combine a traditional spinning platter for mass storage and solid state flash memory for frequently accessed data have always been an interesting concept. They may be slower than SSDs, but not by much, and they are a lot cheaper gigabyte-for-gigabyte. CNET's Harry McCracken speculates on how soon such drives may become mainstream: 'So why would the new Momentus be more of a mainstream hit than its predecessor? Seagate says that it's 70 percent faster than its earlier hybrid drive and three times quicker than a garden-variety, non-hybrid disk. Its benchmarks for cold boots and application launches show the new drive to be just a few seconds slower than a SSD. Or, in some cases, a few seconds faster. In the end, hybrid drives are compromises, neither as cheap as ordinary drives — you can get a conventional 750GB Momentus for about $150 — nor as fast and energy-efficient as SSDs.'"
If there is to be a time for hybrid drives, the window on it is fast closing. As SSDs get cheaper and cheaper more and more people will opt to just go that route. Most people don't really need massive HDDs and so if smaller SSDs get cheap enough that'll be the way they'll go. They don't have to be as cheap as HDDs, just cheap enough that for the size people need (probably 200-300GB for more people) they are affordable enough.
For me personally, the time already came and went. I was very enthusiastic about the concept of hybrid drives, particularly since I have vast storage needs (I do audio production). However no hybrid drive for desktops was forthcoming. Then there was a sale on SSDs, 256GB drives for $200. I picked up two of them. $1/GB was my magic price when I'd be willing to get them. Now I have 512GB of SSD storage for OS, apps, and primary data. That is then backed by 3TB of HDD storage for media, samples, and so on.
A hybrid drive has no place. I'd certainly not replace my SSDs, they are far faster than any hybrid drive (even being fairly slow on the SSD scale). Likewise I have no real reason to upgrade my HDDs, they serve the non-speed intensive stuff.
While I'm willing to spend more than most, it is still a sign of things to come. As those prices drop more and more people will say "screw it" and go all SSD.
There are only two things drive cache can help with significantly. When rebooting, where memory is empty, you can get memory primed with the most common parts of the OS faster if most of that data can be read from the SSD. Optimizers that reorder the boot files will get you much of the same benefit if they can be used.
Disk cache used for writes is extremely helpful, because it allows write combining and elevator sorting to improve random write workloads, making them closer to sequential. However, you have to be careful, because things sitting in those caches can be lost if the power fails. That can be a corruption issue on things that expect writes to really be on disk, such as databases. Putting some flash to cache those writes, with a supercapacitor to ensure all pending writes complete on shutdown, is a reasonable replacement for the classic approach: using a larger battery-backed power source to retain the cache across power loss or similar temporary failures. The risk with the old way is that the server will be off-line long enough for the battery to discharge. Hybrid drives should be able to flush to SSD just with their capacitor buffer, so you're consistent with the filesystem state, only a moment after the server powers down.
As for why read caching doesn't normally help, the operating system filesystem cache is giant compared to any size it might be. When OS memory is gigabytes and drive ones megabytes, you'll almost always be in a double-buffer situation: whatever is in the drive's cache will also still be in the OS's cache, and therefore never be requested. The only way you're likely to get any real benefit from the drive cache is if the drive does read-ahead. Then it might only return the blocks requested to the OS, while caching ones it happened to pass over anyway. If you then ask for those next, you get them at cache speeds. On Linux at least, this is also a futile effort; the OS read-ahead is also smarter than any of the drive logic, and it may very well ask for things in that order in the first place.
One relevant number for improving read speeds is command queue depth. You can get better throughput by ordering reads better, so they seek around the mechanical drive less. There's a latency issue here though--requests at the opposite edge can starve if the queue gets too big--so excessive tuning in that direction isn't useful either.
Hybrid drives, and even all of the hybrid RAID controllers I've looked at, only use the SSD for read acceleration. They aren't used for writes, from what I could tell from their specs. So you're almost certainly better off upgrading your system to the next larger amount of RAM rather than getting a hybrid drive.
Personally, I looked at my storage usage and realized that if I didn't keep *EVERYTHING* on my laptop (every photo I'd taken for 10+ years, 4 or 5 Linux ISOs, etc) and instead put those on a server at home, I could go from a 500GB spinning disc to an 80GB SSD. So I did and there's been no looking back. The first gen Intel X-25M drives had some performance issues, but since then I've been happy with the performance of them.
This Drive is CRAP
ASSUMING that it still only does read caching.
I bought one of the Gen-1 drives and was very underwhelmed. I wanted write caching; 4GB of non-volatile memory with the performance of SLC flash could allow windows (or whatever) to write to the drive flat out for up many seconds without a single choke due to the drive.
In addition 4G of write-back cache is enough to give a significant performance boost for continuous random writes across the drive and even more so across a small extent such as a database or a DotNET native image cache.
But for reading it's insignificant compared to the 3-16Gbytes of (so much faster) main memory that most systems contain, except at boot time when, unlike RAM, it will already contain some data. The problem with this is that it will contain the most recently read data, whereas the boot files can quite reasonably be described as least recently read.
So in the real world it's useless for anything except a machine that's rebooted every five minutes ...
It sucks but has an easy but time consuming fix that leaves you with the drive contents intact:
Boot a live Linux distro. And hook a USB HDD to the system and mount it. The USB hdd can even be formatted NTFS if the live distro has FUSE installed along with the ntfs-3g driver, most live distros already have it or will allow you to install them. Assuming your SSD is the primary or only disk in your system then:
(You need to be root or use sudo, on most live distros you simply type "su root" or "sudo -s")
/dev/sda is the first disk in the system. you may have to run ls /dev/sd* to get a list of disks and partitions. and note, sda is the entire disk block-for-block, sda1 is a partition just like sda2 , sda3 etc. If you have more than one disk and don't know which letter it is then simply type fdisk -lu /dev/sdX (X being the letter you want to check) and it will dump the drive info.
It may take about 5+ hours assuming you have a 512GB SSD and an optimal USB transfer rate of 25MBps to the backup disk (in my experience the average for USB 2.0 write speeds). Faster backup disks and smaller capacity SSD's will backup much faster.
Once complete, you now have a bit for bit block-level copy of the SSD. This ignores the boot sector, boot loaders, partitions and file systems. It does not matter what OS you had on it, how many partitions or what file system you used. if your very paranoid and want to wait hours more, the run diff against the disk and disk image file to be sure they are an exact copy (never did it and never will).
Now reboot and upgrade the firmware the way the manufacturer tells you. So now your data is wiped out, big stinkin deal. Fire up the live Linux distro and again attach your backup disk and then enter the following command:
This writes the image file back to the SSD and if all goes well (It has never failed me yet and I have done this dozens of times for various systems) you now have your upgraded firmware with its original contents fully intact.
You can even mount one or more of the partitions contained within the disk image (under Linux of course) if you do a bit of homework (search google for mounting dd images) or just go here:http://darkdust.net/writings/diskimagesminihowto That tutorial is how I started playing with dd images.
You can also movethe contents of a smaller cramped disk to larger drives. Works for windows/NTFS too! You simply dd the entire smaller drive to the new drive (works best when both drives are hooked up via sata.) Then you use gparted or some other parted disk GUI to grow the file system on the new drive. Shut down and remove the linux cd/thumb-drive and remove the old disk and move the sata cable from the old disk to the new disk. Boot your PC and if your using windows (2000, XP , Vista, 7) it will run the check disk to verify the volume (DONT SKIP IT!) and reboot. Once it reboots to windows, open up explorer and see that you now magically have all that shiny new space without formatting, reinstalling, adding new drive letters or mounting drives under folders etc. Its transparent!
Example command:
sda is the small disk and sdb is the new large disk. I have done that trick multiple times as well with a 100% success rate. My friends were amazed.