RAID 5 Gets Very Slow Very Quickly

I'm new to RAID so please forgive any omissions.

I just setup a new RAID 5 with 4x 2TB drives on a PCI RAID card in Win7 64. I set them up in the BIOS and formatted a GPT partition with no issues. When I started to transfer my data from my old 1TB green drives it would start at ~40 MB/s for the 1st 500mb or so then quickly drop to about 2 MB/s. Since I ordered two of the RAID cards I swapped them out and the 2nd one wasn't much better. It would start at ~95 and drop to 4. Changing nothing I then transfered some files between two of the 1tb drives connected to the mobo at a steady 70 MB/s.

Is this just an issue of junk RAID cards? Do I need to go to PCI-Express for decent speeds? Any help would be appreciated.


Win 7 64
Athlon 64 x2
4gb DDR
GA-K8N-SLi (nForce 4)

4x WD Green AV 2TB
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  1. FakeRAID5 is both unreliable and very slow particularly when writing. Do not consider this anything more reliable than a single disk, and force yourself to keep extremely tight backups when using less reliable RAID solutions.

    SiI-3124 is a PCI-X chip, if you're using it on PCI the performance will be even worse.

    Consider avoiding RAID5 and use the onboard RAID capabilities or no RAID at all; all these fakeRAID solutions (nVidia MediaShield, Silicon Image) are not very reliable at all.
  2. What makes it "fake" and how do I get real RAID5 then? I can afford to spend a little more on a nice card if I have to. I've been running a media server for almost 10 years now and HDD failure is by far the most common way I loose data. I really need a good solution that still maximizes my available storage. With 5TB of data and counting backup isn't really a viable solution. The best I can do there is fill up drives for friends from time to time so I can get things back from them in case of a failure.
  3. FakeRAID stems from the fact that many people think the onboard RAID is hardware RAID; while in fact the essence is 100% software; the Windows-only drivers do the RAID part; there is no hardware RAID on your motherboard. The only hardware is the SATA controller and a small CMOS which stores tiny software used for bootstrapping; otherwise Windows could not boot from the RAID array. But this takes about a second, then it transfers control to the Windows storage driver instead.

    So onboard RAID is driverRAID or softwareRAID; and absolutely not hardware RAID. That's the reason it has gotten the "FakeRAID" name. Onboard RAID generally is of low quality, has several reliability issues and the performance is not too great as well.

    RAID5 is complicated, and generally i suggest you avoid it. Though it sounds great: you sacrifice only one drive for parity data and add some protection to your files. But the RAID layer itself reduces the reliability of the storage solution; the RAID engine itself is a point of failure you introduce; an extra component that can fail. And they DO fail, quite regularly!

    In fact, most onboard RAIDs fail not due to physical disk failure, but rather failures of the RAID engine themselve! A common type of failure is a bad sector; non-TLER disks will be detached/failed resulting in a broken/split array, and it is quite common for users to lose their data even though the disks themselves were just fine; just a small hickup is enough to trigger bad things and without proper user interaction, you can lose all the data on the RAID.

    My advice is: invest in something that IS reliable, such as ZFS fileserver. If you can't then invest in a very good BACKUP rather than RAID. Consider RAID on Windows convenient so you have one big volume, but not very reliable; no matter what RAID level. Especially RAID5/6 i would consider less reliable than a single disk without RAID; even though it grants you protection in theory; many people discount the theoretical risk. Using RAID both increases and decreases reliability; that's the cold truth.

    So the best advice i can give you is: invest in a backup. Make two computers on the LAN in your home backup eachother, simple yet effective. External disks may also help you. And have a look at ZFS if you want something advanced and reliable.
  4. Two comments to add to sub mesa's post:

    1) Don't expect RAID 5 to perform well for writing, even if you buy a hardware RAID controller. No RAID 5 system can perform very well for sustained writes because RAID 5 requires a read-modify-write operation on two drives every time you write data.

    2) Large RAID 5 arrays like the type you're using, even with a hardware controller, are a bad idea. The problem is that if a disk fails, the only way to recover all the data is to successfully read EVERY sector from EVERY other drive. For large arrays this takes a LONG time (days) during which performance is VERY poor, and during that time the chance of another drive failing or encountering an unrecoverable sector is fairly high (the more drives, the higher the probability). So as sub mesa says, for large volumes RAID 5 reliability is really not something you can rely on.
  5. +1 for above 2 posts. I have not extensively tested this, but you can try Windows Software RAID that is built in to Windows 7 (assuming Pro or higher). I am currently testing/trying Windows RAID1/0 on my test bed. I don't have a 3rd spare drive to test RAID5.

    Btw, I'm talking about the actual Windows RAID here, not Fake RAID. As in this:

    However, like sub said. You are better off with a NAS and a good back up system.RAID is NOT a back up system.

    If you want TRUE RAID5 prepare to shell out a couple of hundred or more for a enterprise RAID card.

    You could also look in to RAID01 since you have 4x HDDs. This should be able to deliver the speeds at the cost of space.

    Also realize that WD "Green" drives are slow to begin with.

    @OP: You may actually be one of the few people who may want to consider BD as a 2nd/3rd level back up if you really do have frequent HDD failures. You can also look in to online "cloud" storage solutions. If you do plan to use the "cloud" I recommend you encrypt your data before storing online. Personally, I use Dropbox to store my important files. I just create a TrueCrypt container and put the data there and upload the container. I'v been doing this for a few years no. No problems so far. Yes, this takes time, but it is worth it imo.
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