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Thread: fragmentation - what does this mean and how will SSDs effect linux

  1. #1
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    fragmentation - what does this mean and how will SSDs effect linux

    hi folkes,

    so I thought that this might be interesting for discussion and i curious about the truth of the matter, so here goes (this is my understanding):

    External Fragmentation: this is what NTFS / windows suffers from and is where dynamically allocated memory chunks are freed, however are no longer usable as a result of their non-standard small size. also i think the NTFS system preferentially stores data in the 'next' space on the HDD ie consecutively, like a big stack.

    Internal Fragmentation: this is what Linux suffers from inodes are use to create linked trees of data each of the nodes and leaves are a standardized size therefore when they are freed the space can easily be used by another program. this gets rid of any eternal fragmentation however as a result of the standard size of memory blocks there is typically unused memory which is allocated to each program and not usable by any other program. like each customer in a store having a basket with unused space in it, the store has only so many baskets, and the customers cannot share their unused basket space.

    SSDs: apparently these do not suffer from fragmentation (i don't know how, but this is what i heard). this is presumably referring to external fragmentation. does this mean that inodes and the memory organization used by linux are now a weakness (as the fragmentation is logical and therefore still applicable).

    any help on understanding this issue are most welcome.

    edit - so I just realised that the title should read 'affect', not 'effect'. sorry to all you grammarians.
    Last edited by F.G.; October 29th, 2011 at 02:30 PM. Reason: spelling

  2. #2
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    Re: fragmentation - what does this mean and how will SSDs effect linux

    SSDs do not benefit from defragmentation because there is little benefit to reading data sequentially (beyond typical FS block sizes) and any defragmentation process adds additional writes on the NAND flash that already have a limited cycle life

  3. #3
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    Re: fragmentation - what does this mean and how will SSDs effect linux

    ok, this is pretty much what i thought, but still supports the idea that fixed size memory allocation is no longer useful with SSDs.

    also about the limited life cycle, i read in another thread that you could write continuously to a SSD for 40 years before exhausting this, and this as a practical issue is essentially mythological.

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    Re: fragmentation - what does this mean and how will SSDs effect linux

    Quote Originally Posted by F.G. View Post
    also about the limited life cycle, i read in another thread that you could write continuously to a SSD for 40 years before exhausting this, and this as a practical issue is essentially mythological.
    The life of a ssd exceeds that of normal hard drives so I don't see the issue.

  5. #5
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    Re: fragmentation - what does this mean and how will SSDs effect linux

    SSDs need defragmentation, but the defrag algorithms for most filesystems only works for normal spinning disks, meaning that a lot of information would be poorly and wrongly allocated things on the SSD if defragged by most applications. That means that after defragmention, the cleanup algorythm on the SSD would need to reallocated almost everything, resulting in weardown and time used for this move.
    It also results in a slowdown of writing to the disk because of the way the allocation is done.

    Perhaps LogFS and some other flash filesystems are good enough soon, we nobody will sit down and benchmark them to figure that out.

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    Re: fragmentation - what does this mean and how will SSDs effect linux

    Quote Originally Posted by Gremlinzzz View Post
    already have a limited cycle life
    If you consider writing to the disk 24/7 for at least 8 years limited, then yes. At this point, I would trust an ssd before a mechanical HD.

  7. #7
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    Re: fragmentation - what does this mean and how will SSDs effect linux

    hey, so i saw this link in another thread:

    http://elitepcbuilding.com/ssd-vs-hdd

    it indicates that that SSDs can sustain continuous writes (24/7) for 40 years (rather than 8) which is easily enough for me.
    though this isn't really what this thread was asking about, what i'm wondering about is fixed vs dynamic memory allocation inodes and fragmentation in SSDs.

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    Re: fragmentation - what does this mean and how will SSDs effect linux

    SSD drives should remain heavily fragmented due to how they store data... there is a leveling feature of the drive which helps push the lifespan out to 10+ years. Defragmenting the drive will kill it over time. It stores the data like a water table.

  9. #9
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    Re: fragmentation - what does this mean and how will SSDs effect linux

    Isn't F.G.'s OP correct, though? SSDs don't have a problem with fragmentation, since the physical location doesn't matter, but Linux filesystems still take more space on disk to store the same amount of data, as a result of being designed to avoid fragmentation. It doesn't really matter, of course, but it's a fair observation.

    Windows isn't using NTFS in 8, though, so ext4 vs. NTFS isn't really important.

  10. #10
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    Re: fragmentation - what does this mean and how will SSDs effect linux

    Do SSDs Slow Down Over Time?Yes. But that's just the short answer. Solid state drives, or SSDs, have often been hailed as a fast, cheap future for memory... but they have one critical flaw: a gradual slow down and corruption over time. Here's an overview of the problem, and some of the things being down to alleviate this.

    http://www.brighthub.com/computing/h...les/43400.aspx



    Defragmenting or "defragging" a SSD takes up many write/erase cycles... which shortens the lifetime of an SSD,
    Last edited by Gremlinzzz; October 29th, 2011 at 08:59 PM.

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