Why SSDs Care About What You Write: Fragmentation & Write Combining

PC Perspective's Allyn Malventano is a smart dude, just read one of his articles to figure that out. He pieced together a big aspect of how the X25-M worked on his own, a major key to how to improve SSD performance.

You'll remember from the Anthology that SSDs get their high performance by being able to write to multiple flash die across multiple channels in parallel. This works very well for very large files since you can easily split the reads and writes across multiple die/channels.

Here we go to write a 128KB file, it's split up and written across multiple channels in our tiny mock SSD:

When we go to read the file, it's read across multiple channels and performance is once again, excellent.

Remember what we talked about before however: small file random read/write performance is actually what ends up being slowest on hard drives. It's what often happens on a PC and thus we run into a problem when performing such an IO. Here we go to write a 4KB file. The smallest size we can write is 4KB and thus it's not split up at all, it can only be written to a single channel:

As Alyn discovered, Intel and other manufacturers get around this issue by combining small writes into larger groups. Random writes rarely happen in a separated manner, they come in bursts with many at a time. A write combining controller will take a group of 4KB writes, arrange them in parallel, and then write them together at the same time.

This does wonders for improving random small file write performance, as everything completes as fast as a larger sequential write would. What it hurts is what happens when you overwrite data.

In the first example where we wrote a 128KB file, look what happens if we delete the file:

Entire blocks are invalidated. Every single LBA in these blocks will come back invalid and can quickly be cleaned.

Look at what happens in the second example. These 4KB fragments are unrelated, so when one is overwritten, the rest aren't. A few deletes and now we're left with this sort of a situation:

Ugh. These fragmented blocks are a pain to deal with. Try to write to it now and you have to do a read-modify-write. Without TRIM support, nearly every write to these blocks will require a read-modify-write and send write amplification through the roof. This is the downside of write combining.

Intel's controller does its best to recover from these situations. That's why its used random write performance is still very good. Samsung's controller isn't very good at recovering from these situations.

Now you can see why performing a sequential write over the span of the drive fixes a fragmented drive. It turns the overly fragmented case into one that's easy to deal with, hooray. You can also see why SSD degradation happens over time. You don't spend all day writing large sequential files to your disk. Instead you write a combination of random and sequential, large and small files to the disk.

The Cleaning Lady and Write Amplification A Wear Leveling Refresher: How Long Will My SSD Last?
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  • Wwhat - Sunday, September 06, 2009 - link

    If you read the first part of the article alone you would see how important a good controller is in a SSD and you would no ask his question probably, plus SSD's use the flash in parallel where a bunch of USB drives would not, the parallel thing is also mentioned in the article.
    And USB has a lot of overhead actually on the system, both in CPU cycles as well as in IO interrupts.

    There are plug in PCI(e) cards to stick SD cards in though, to get a similar setup, but it's a bit of a hack and with the overhead and the management and controllers used and the price to buy many SD cards it's not competitive in the end and you are better of with a real SSD I'm told.
    Reply
  • Transisto - Sunday, September 06, 2009 - link

    You are right, the controller is very important.

    I think caching about 4-8 gig of most often accessed program files has the best price/performance ratio, for improving application load time. It it also very easily scalable.

    One of the problem I see is integrating this ssd cache in the OS or before booting so it act where it matter the most.

    I think there could be a near x25-m speedup from optimized caching and good controller no matter what SSD form factor it rely on. SD, CF, usb , pci or onboard.

    Why it seams nobody talk about eboostr type of caching AND ,,, on other news ,,, Intel's Braidwood flash memory module could kill SSD market.

    I am quite of a performance seeker.

    But I don't think I need 80gig of SSD in my desktop,just some 8gb of good caching. Mabe a 60gb ssd on a laptop.

    Well... I'm gonna pay for that controller once, not twice (160gb?)
    Reply
  • Wwhat - Saturday, September 05, 2009 - link

    Not that it's not a good article, although it does seem like 2 articles in one, but what I miss is getting to brass tacks regarding the filesystem used, and why there isn't a SSD-specific file system made, and what choices can be made during formatting in regards to blocksize, obviously if you select large blocks on filesystem level a would impact he performance of the garbage collection right? It actually seem the author never delved very deeply into filesystems from reading this.
    The thing is that even with large blocks on filesystem level the system might still use small segments for the actuall keepin track, and if it needs to write small bits to keep track of large blocks you'd still have issues, that's why I say a specific SSD filesystem migh be good, but only if there isn't a new form of SSD in the near future that makes the effort poinless, and if a filesystem for SSD was made then the firmware should not try to compensate for exising filesystem issues with SSD's.
    I read that the SD people selected exFAT as filesystem for their next generation, and that also makes me wonder, is that just to do with licensing costs or is NTFS bad for flash based devices?
    Point being at the filesystem needs to be highlighted more I think,
    Reply
  • Bolas - Friday, September 04, 2009 - link

    Would someone please hit Dell with the clue-board and convince them to offer the Intel SSD's in their Alienware systems? The Samsung SSD's are all that is stopping me from buying an Alienware laptop at the moment. Reply
  • EatTheMeat - Friday, September 04, 2009 - link

    Congratulations on another fab masterclass. This is easily the best educational material on the internet regarding SSDs, and contrary to some comments, I think you've pitched your recommendations just right. I can also appreciate why you approached this article with some trepidation. Bravo.

    I have a RAID question for Anand (or anyone else who feels qualified :-))

    I'm thinking of setting up 2 160GB x25-m G2 drives in RAID-0 for Win 7. I'd simply use the ICH10R controller for it. It's not so much to increase performance but rather to increase capacity and make sure each drive wears equally. After considering it further I'm wondering if SSD RAID is wise. First there's the eternal question of stripe size and write amplification. It makes sense to me to set the stripe size to be the same as, or a fraction of, the block size of the SSD. If you choose the wrong stripe size does it influence write amplification?

    I'm aware that performance should increase with larger spripes, but I'm more concerned about what's healthy for the SSD.

    Do you think I should just let SSD RAID wait until RAID drivers are optimised for SSDs?

    I know you're planning a RAID article for SSDs - I for one look forward to it greatly. I've read all your other SSD articles like four times!
    Reply
  • Bolas - Friday, September 04, 2009 - link

    If SSD's in RAID lose the benefit of the TRIM command, then you're shooting yourself in the foot if you set them up in RAID. If you need more capacity, wait for the Intel 320GB SSD drives next year. Or better yet, use a 160 GB for your boot drive, then set up some traditional hard disk drives in RAID for your storage requirements. Reply
  • EatTheMeat - Friday, September 04, 2009 - link

    Thanks for reply. I definitely hear you about the TRIM functionality as I doubt RAID drivers will pass this through before 2010. Still though, it doesn't look like the G2s drop much in performance with use anyway from Anand's graphs. With regard to waiting for 320 GB drives - I can't. These things are just too enticing, and you could always say that technology will be better / faster / cheaper next year. I've decided to take the plunge now as I'm fed up with an i7 965 booting and loading apps / games like a snail even from a RAID drive.

    I just don't want to bugger the SSDs up with loads of write amplification / fragmentation due to RAID-0. ie, is RAID-0 bad for the health of SSDs like defragmentation / prefetch is? I wonder if anyone knows the answer to this question yet.
    Reply
  • jagreenm - Saturday, September 05, 2009 - link

    What about just using Windows drive spanning for 2 160's? Reply
  • EatTheMeat - Saturday, September 05, 2009 - link

    As far as I know drive spanning doesn't even the wear between the discs. It just fills up first one and then the other. That's important with SSDs because RAID can really help reduce drive wear by spreading all reads and writes across 2 drives. In fact, it should more than half drive wear as both drives will have large scratch portions. Not so with spanning as far as I know.

    Does anyone know if I'm talking sh1t here? :-)
    Reply
  • pepito - Monday, November 16, 2009 - link

    If you are not sure, then why do you assert such things?

    I don't know about Windows, but at least in Linux when using LVM2 or RAID0 the writes spread evenly against all block devices.
    That means you get twice the speed and better drive wear.

    I would like to think that microsoft's implementation works more or less the same way, as this is completely logical (but then again, its microsoft, so who can really know?).
    Reply

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