New vs Used SSD Performance

We begin our look at how the overhead of managing pages impacts SSD performance with iometer. The table below shows iometer random write performance; there are two rows for each drive, one for “new” performance after a secure erase and one for “used” performance after the drive has been well used.

4KB Random Write Speed New "Used"
Intel X25-E   31.7 MB/s
Intel X25-M 39.3 MB/s 23.1 MB/s
JMicron JMF602B MLC 0.02 MB/s 0.02 MB/s
JMicron JMF602Bx2 MLC 0.03 MB/s 0.03 MB/s
OCZ Summit 12.8 MB/s 0.77 MB/s
OCZ Vertex 8.2 MB/s 2.41 MB/s
Samsung SLC 2.61 MB/s 0.53 MB/s
Seagate Momentus 5400.6 0.81 MB/s -
Western Digital Caviar SE16 1.26 MB/s -
Western Digital VelociRaptor 1.63 MB/s -

 

Note that the “used” performance should be the slowest you’ll ever see the drive get. In theory, all of the pages are filled with some sort of data at this point.

All of the drives, with the exception of the JMicron based SSDs went down in performance in the “used” state. And the only reason the JMicron drive didn’t get any slower was because it is already bottlenecked elsewhere; you can’t get much slower than 0.03MB/s in this test.

These are pretty serious performance drops; the OCZ Vertex runs at nearly 1/4 the speed after it’s been used and Intel’s X25-M can only crunch through about 60% the IOs per second that it did when brand new.

So are SSDs doomed? Is performance going to tank over time and make these things worthless?


"Used" SSD performance vs. conventional hard drives.

Pay close attention to the average write latency in the graph above. While Intel’s X25-M pulls an extremely fast sub-0.3ms write latency normally, it levels off at 0.51ms in its used mode. The OCZ Vertex manages a 1.43ms new and 4.86ms used. There’s additional overhead for every write but a well designed SSD will still manage extremely low write latencies. To put things in perspective, look at these drives at their worst compared to Western Digital’s VelociRaptor.The degraded performance X25-M still completes write requests in around 1/8 the time of the VelociRaptor. Transfer speeds are still 8x higher as well.

Note that not all SSDs see their performance drop gracefully. The two Samsung based drives perform more like hard drives here, but I'll explain that tradeoff much later in this article.

How does this all translate into real world performance? I ran PCMark Vantage on the new and used Intel drive to see how performance changed.

PCMark Overall Score New "Used" % Drop
Intel X25-M 11902 11536 3%
OCZ Summit 10972 9916 9.6%
OCZ Vertex 11253 9836 14.4%
Samsung SLC 10143 9118 10.1%
Seagate Momentus 5400.6 6817 - -
Western Digital VelociRaptor 7500 - -

 

The real world performance hit varies from 0 - 14% depending on the drive. While the drives are still faster than a regular hard drive, performance does drop in the real world by a noticeable amount. The trim command would keep the drive’s performance closer to its peak for longer, but it would not have prevented this from happening.

PCMark Vantage HDD Test New "Used" % Drop
Intel X25-M 29879 23252 22%
JMicron JMF602Bx2 MLC 11613 11283 3%
OCZ Summit 25754 16624 36%
OCZ Vertex 20753 17854 14%
Samsung SLC 17406 12392 29%
Seagate Momentus 5400.6 3525 -  
Western Digital VelociRaptor 6313 -  

 

HDD specific tests show much more severe drops, ranging from 20 - 40% depending on the drive. Despite the performance drop, these drives are still much faster than even the fastest hard drives.

Simulating a Used Drive SSD Aging: Read Speed is Largely Unaffected
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  • tirez321 - Wednesday, March 18, 2009 - link

    I can kinda see that it wouldn't now.
    Because there would still be states there regardless.
    But if you could inform the drive that it is deleted somehow, hmm.

    Reply
  • strikeback03 - Wednesday, March 18, 2009 - link

    The subjective experiences with stuttering are more important to me than most of the test numbers. Other tests I have found of the G.Skill Titan and similar have looked pretty good, but left out mention of stuttering in use.

    Too bad, as the 80GB Intel is too small and the ~$300 for a 120GB is about the most I am willing to pay. Maybe sometime this year the OCZ Vertex or similar will get there.
    Reply
  • strikeback03 - Tuesday, March 24, 2009 - link

    When I wrote that, the Newegg price for the 120GB Vertex was near $400. Now they have it for $339 with a $30 MIR. Now that's progress. Reply
  • kamikaz1k - Wednesday, March 18, 2009 - link

    the latency times are switched...incase u wanted to kno.
    also, first post ^^ hallo!
    Reply
  • GourdFreeMan - Wednesday, March 18, 2009 - link

    It seems rather premature to assume the ATA TRIM command will significantly improve the SSD experience on the desktop. If you were to use TRIM to rewrite a nonempty physical block, you do not avoid the 2ms erase penalty when more data is written to that block later on and instead simply add the wear of another erase cycle. TRIM, then, is only useful for performance purposes when an entire 512 KiB physical block is free.

    A well designed operating system would have to keep track of both the physical and logical maps of used space on an SSD, and only issue TRIM when deletion of a logical cluster coincides with the freeing of an entire physical block. Issuing TRIMs at any other time would only hurt performance. This means the OS will have significantly fewer opportunities to issue TRIMs than you assume. Moreover, after significant usage the physical blocks will become fragmented and fewer and fewer TRIMs will be able to be issued.

    TRIM works great as long as you only deal with large files, or batches of small files contiguously created and deleted with significant temporal locality. It would greatly aid SSDs in the "used" state Anand artificially creates in this article, but on a real system where months of web browsing, Windows updates and software installing/uninstalling have occurred the effect would be less striking.

    TRIM could be mated with periodic internal (not filesystem) defragmentation to mitigate these issues, but that would significantly reduce the lifespan of the SSD...

    It seems the real solution to the SSD performance problem would be to decrease the size of the physical block... ideally to 4 KiB, as that is the most common cluster size on modern filesystems. (This assumes, of course, that the erase, read and write latencies could be scaled down linearly.)
    Reply
  • Kary - Thursday, March 19, 2009 - link

    Why use TRIM at all?!?!?

    If you have extras Blocks on the drive (NOT PAGES, FULL BLOCKS) then there is no need for TRIM command.

    1)Currently in use BLOCK is half full
    2)More than half a block needs to be written
    3)extra BLOCK is mapped into the system
    4)original/half full block is mapped out of system.. can be erased during idle time.

    You could even bind multiple continuous blocks this way (I assume that it is possible to erase simultaneously any of the internal groupings pages from Blocks on up...they probably share address lines...ex. erase 0000200 -> just erase block #200 ....erase 00002*0 -> erase block 200 to 290...btw, did addressing in base ten instead of binary just to simplify for some :)
    Reply
  • korbendallas - Wednesday, March 18, 2009 - link

    Actually i think that the Trim command is merely used for marking blocks as free. The OS doesn't know how the data is placed on the SSD, so it can't make informed decision on when to forcefully erase pages. In the same way, the SSD doesn't know anything about what files are in which blocks, so you can't defrag files internally in the drive.

    So while you can't defrag files, you CAN now defrag free space, and you can improve the wear leveling because deleted data can be ignored.

    So let's say you have 10 pages where 50% of the blocks were marked deleted using the Trim command. That means you can move the data into 5 other pages, and erase the 10 pages. The more deleted blocks there are in a page, the better a candidate for this procedure. And there isn't really a problem with doing this while the drive is idle - since you're just doing something now, that you would have to do anyway when a write command comes.
    Reply
  • GourdFreeMan - Wednesday, March 18, 2009 - link

    This is basically what I am arguing both for and against in the fourth paragraph of my original post, though I assumed it would be the OS'es responsibility, not the drive's.

    Do SSDs track dirty pages, or only dirty blocks? I don't think there is enough RAM on the controller to do the former...
    Reply
  • korbendallas - Wednesday, March 18, 2009 - link

    Well, let's take a look at how much storage we actually need. A block can be erased, contain data, or be marked as trimmed or deallocated.

    That's three different states, or two bits of information. Since each block is 4kB, a 64GB drive would have 16777216 blocks. So that's 4MB of information.

    So yeah, saving the block information is totally feasible.
    Reply
  • GourdFreeMan - Thursday, March 19, 2009 - link

    Actually the drive only needs to know if the page is in use or not, so you can cut that number in half. It can determine a partially full block that is a candidate for defragmentation by looking at whether neighboring pages are in use. By your calculation that would then be 2 MiB.

    That assumes the controller only needs to support drives of up to 64 GiB capacity, that pages are 4 KiB in size, and that the controller doesn't need to use RAM for any other purpose.

    Most consumer SSD lines go up to 256 GiB in capacity, which would bring the total RAM needed up to 8 MiB using your assumption of a 4 KiB page size.

    However, both hard drives and SSDs use 512 byte sectors. This does not necessarily mean that internal pages are therefore 512 bytes in size, but lacking any other data about internal pages sizes, let's run the numbers on that assumption. To support a 256 MiB drive with 512 byte pages, you would need 64 MiB of RAM -- which only the Intel line of SSDs has more than -- dedicated solely to this purpose.

    As I said before there are ways of getting around this RAM limitation (e.g. storing page allocation data per block, keeping only part of the page allocation table in RAM, etc.), so I don't think the technical challenge here is insurmountable. There still remains the issue of wear, however...
    Reply

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