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Confused on Storage Spaces with Parity and free space

  1. #1

    Confused on Storage Spaces with Parity and free space


    Let's get this clear right at the start, I manage servers for a living and I understand RAID arrays and storage pretty well. i know the differences between RAID 0,1,5,6,10,50, etc.

    In playing with Storage Spaces on Windows 8 and using a resiliency type of parity has left me confused as to how the drives are filling up as fast as what they are.

    In my lab, I created a Windows 8 VM, with 3 x 4GB additional drives. So, 12GB of RAW capacity between the 3 drives.

    CREATING THE STORAGE POOL
    So, I created a Storage Pool using all 3 drives and am left with a total pool capacity of 9.75GB. It seems that about 750MB of each physical drive is consumed by overhead, leaving me with actual usable space of 3.25GB per drive.
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    CREATING THE STORAGE SPACE WITH PARITY
    So, I created a single space, used Parity for the resiliency type. I set the size to 6GB, so that including the parity information, only 9GB would be consumed. Thus, I'm not using "thin provisioning", but instead ensuring that I have enough capacity to actually store my data.

    Click image for larger version

    A LOOK AT THE POOL AND SPACE WHEN DRIVE IS EMPTY
    My confusion starts here. Even though I'm not yet storing a single file, approx 35% of my pool is being used. So, considering it's parity, and RAID (Raid 3 specifically from what I have read), I have to assume that it's just visually showing me that 1 out of my 3 drives will be used for parity, and thus nearly 33% will be consumed by parity.
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    COPY A SINGLE 1GB FILE TO MY NEW STORAGE SPACE
    So, I copy in 1 single 1GB ISO file and place it into my Storage Space.
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    A look at my storage pool, leaves me confused. My pool usage went from 3.50GB used to 5.75GB used because of 1 single 1GB file????? If it wrote say 500MB to 1 drive, and 500MB to a 2nd drive, and consumed 500MB in parity on a 3rd drive...that would be 1.5GB....so I'm unsure how 2.25GB of space was reportedly used.

    Click image for larger version

    COPY A FEW MORE FILES TO THE STORAGE SPACE (Approx 4GB worth)
    So, I copy some more files into my Storage Space. It's 7 files and totals 4GB of space
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    Here is the Properties of my Storage Space drive in Explorer;

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    Explorer looks right, approx 1.8GB free out of my 6GB volume (after copying in 4GB of files)

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    However, my Storage Pool is pretty much full. Huh? How can this be, I've only copied 4GB of files into a 6GB volume. How can it possibly be out of space??
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    Summary
    3 x 4GB setup would provide 12GB of RAW space (say, if I did RAID 0)

    With standard RAID 5 on a server, I would expect 2 x 4GB or so usable (around 8GB), with 4GB dedicated to parity

    With Windows 8 Storage Spaces, I setup 6GB for what I thought would be "usable", leaving 3GB for parity and leaving around .75GB still free in the pool.

    But yet, after copying 4GB of data, I'm maxed out. And trying to copy any more data to my Storage Space takes the Space "OFFLINE" as it's full.

    So, it seems 4GB out of 12GB is usuable. So, it's 33% data, and 66% parity and overhead. How can this be this inefficient? A parity setup should provide more "usable" space than a 2-way mirror....which would give me 50%.

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  2. #2


    Adding Insult to Injury, I tried a 2-way MIRROR with Storage Spaces

    Added 2 x 6GB drives to my machine, created a New Pool, created a new 2 way Storage Space pool. As expected, .75GB on each physical drive was unusable as a result of overhead...so my Pool provided 5.25GB on each drive, x 2 drives, so 10.50GB total.

    And since I was using 2 WAY MIRRORING, I expected to only get 5.25GB of total usable space.

    However, after copying the 7 files that are 4GB from above, my storage space went offline and said it was FULL. WTF???

    Click image for larger version

    You can see that Explorer reports things correctly, I have free space. My 2 way mirror drive is not 100% full.
    Click image for larger version
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  3. #3


    So, I think the underlying issue is the way that Storage Spaces allocated space on the physical volumes. The data is striped across "Slabs". The "slabs", are 256MB in size. So, lets say that you need to write a 50MB file. When you using a "2-way-mirror", the system will ask the Storage Pool for 2 "slabs" on different disks and will write the 50MB file into slab 1 and slab 2. So, in theory, even though you added just a single 50MB file....the underlying storage system would have consumed 2 more slabs at 256MB each...so 512MB of space in the pool would be consumed.

    I have to assume in my parity example, that I might have had 200MB free in a slab, but since I needed to store a 1GB file, the system would have asked for 4 more slabs and thus left free space on other slabs that simply couldn't be used because my file was too big.

    Therefore, my Storage Space was actually running out of free slabs to store more files....rather than actually running out of disk space based on the # of slabs already allocated. Had I stored 250MB ISO's which would have fit into a single slab and filled it nearly full...I might have gotten closer to 6GB of data on the Storage Space.

    Oh well, moral of the story is that Storage Spaces should probably be used cautiously and stringing together disks might result in far less actual usable disk space than you think.
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  4. #4


    Posts : 5,707
    Windows 8.1 Pro


    Yeah, I was about to say that the 256MB slabs are probably what's causing it.

    But gee, I'm still trying to wrap my head around the workings of Storage Spaces, I understand RAID pretty well as it's not difficult to get, but Storage Spaces to me is like a new BMW versus a car with a carburetor and choke.
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  5. #5


    Storage spaces isn't much more than an implementation of LVM which other OS vendors have had available for many, many, many years. Their implementation leaves quite a lot to be desired. Hopefully they improve upon it dramatically. Based on what I have seen, I wouldn't use it in any type of production environment. I'd much rather just have standalone disks with their own drive letters.
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  6. #6


    Posts : 5,707
    Windows 8.1 Pro


    Quote Originally Posted by pparks1 View Post
    Storage spaces isn't much more than an implementation of LVM which other OS vendors have had available for many, many, many years. Their implementation leaves quite a lot to be desired. Hopefully they improve upon it dramatically. Based on what I have seen, I wouldn't use it in any type of production environment. I'd much rather just have standalone disks with their own drive letters.
    There kind of does leave some things to be desired. Storage Spaces is most preferable on Server 2012 and ReFS to offer the best safeguard against data loss/corruption.

    Having said that, I'd still use in a production environment. From what I know and have researched (partly thanks to you) Storage Spaces seems a better option over RAID. It's like RAID, but enhanced more or less. And, there's also the fact that ANY interface type of hard drive can be used.

    Although I kind of want ReFS...maybe Windows Blue or 9?
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  7. #7


    Quote Originally Posted by Coke Robot View Post
    Having said that, I'd still use in a production environment. From what I know and have researched (partly thanks to you) Storage Spaces seems a better option over RAID. It's like RAID, but enhanced more or less. And, there's also the fact that ANY interface type of hard drive can be used.
    It's simpler than RAID. However, a hardware based RAID solution is FAR and away better as far as performance is concerned. Storage spaces with Parity is Abysmally slow.
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  8. #8


    Posts : 5,707
    Windows 8.1 Pro


    Quote Originally Posted by pparks1 View Post
    Quote Originally Posted by Coke Robot View Post
    Having said that, I'd still use in a production environment. From what I know and have researched (partly thanks to you) Storage Spaces seems a better option over RAID. It's like RAID, but enhanced more or less. And, there's also the fact that ANY interface type of hard drive can be used.
    It's simpler than RAID. However, a hardware based RAID solution is FAR and away better as far as performance is concerned. Storage spaces with Parity is Abysmally slow.
    Oh yeah this is true! Depends on the RAID version though if you get increased speeds. Storage Spaces seems more like used for archiving data, as well as being chunked into different redundancy options. I've yet to see any parity speed tests, how slow is abysmally slow?
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  9. #9


    Posts : 1,925
    Windows 8.1 Pro


    Storage Spaces is designed to solve a number of problems that RAID doesn't do. If you want RAID, then use RAID. Storage Spaces has similarities, but it is designed differently in order to allow for things that RAID doesn't do.

    For instance, you can't create a volume that's larger than your available capacity in RAID, and then just add more disks as your volumes fill up. In RAID, You have to add more drives to the array, reconfigure it, then expand the volumes.. all this is pretty complicated to do correctly, and very confusing.

    SS is designed as a simple, create a gigantic volume or set of volumes.. and just forget about it until it runs out of disk space, then just pop in more drives, and everything just keeps working. Certainly, it should do a better job of notifying the user when space is getting low, and yes.. it's pretty slow.. but it's designed to be used for long term storage, not for performance. You know, storing your library of ripped movies, for instance.

    As to your confusion about space usage, you're really only seeing that problem because you're working with small drives. If you were talking about multiple terabyte arrays, then this is something you won't even notice.
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  10. #10


    Quote Originally Posted by Coke Robot View Post
    I've yet to see any parity speed tests, how slow is abysmally slow?
    ~15MB/sec. Whereas a 2 way mirror on same hardware gets around 85MB/sec

    Quote Originally Posted by Mystere View Post
    As to your confusion about space usage, you're really only seeing that problem because you're working with small drives. If you were talking about multiple terabyte arrays, then this is something you won't even notice.
    Well, tell that to people who are still complaining that their 2TB hard drive is not 2,000GB, but rather 1.81GB within Windows.

    Sure, my case is exaggerated due to small drives..no doubt about that. But the underlying concern is still a concern. Losing 100GB or so on 2-3TB is small...but it's still a loss.
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Confused on Storage Spaces with Parity and free space
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