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Microsoft Storage Spaces Frequently Asked Questions / Quick Guide
MS Storage Spaces

Revision - 1.0

Author bio: Elena Pakhomova is the co-founder of offering data recovery solutions for various storage devices.

Q: What is Storage Spaces?

A: Storage Spaces is a new technology implemented in Windows 8 which allows you to effectively manage disk space. With Storage Spaces, physical drives are combined into the pools on which then spaces are created that from the user’s point of view look like typical logical drives.The main difference between Storage Spaces and previous storage technologies such as dynamic disks is that Storage Spaces is much more flexible. It is possible to use drives with different sizes and with various type of connection, for example, drives connected via USB.

Q: What are the main design priorities of Storage Spaces? What are the main features of Storage Spaces?

A: The main design priorities are:

  1. Scalability. Storage Spaces can work with a large number of drives. You can use at least 10 times more drives than in the array of dynamic disks.

  2. Flexibility. Storage Spaces allows much more complex configurations and use cases than dynamic disks used in previous versions of Windows.

  3. Microsoft would insist on greater reliability, but the improvement over dynamic disks or hardware controllers seems negligible based on one year of experience. The only noticeable difference is RAID5 write hole protection, coming at a cost of worse RAID5 performance.

The main features are:

  1. Fault tolerance - mirroring, parity, and “hot spare” capabilities.

  2. Storage Spaces allows you to combine several storage devices into one virtual disk. The physical drives can be connected in different ways (SATA or USB) and may be different in size.

Q: Where can I find more information about Storage Spaces?

A: Start with the following references:

Q: What are the disadvantages of Storage Spaces?


  • It is impossible to boot from the drives included in the Storage Spaces pool.

  • Performance of parity spaces is quite low due to the built-in journaling, which cannot be turned off.

  • Implicit limitations on the use of disk capacity. The rules used to set the number of columns for a given space are not exactly obvious. Misunderstanding these can lead to some nasty surprise later on.

  • Lack of automatic rebalancing of disk space between the physical drives when they are filled up unevenly.

  • The complexity of repair and recovery in case of failure.

Q: What is a pool?

A: Pool is a set of physical drives and spaces (which is a Microsoft term for virtual disks) which are placed on these physical drives.

Q: What is a slab?

A: Slab is a minimum quantum of disk space allocated by Storage Spaces on the physical drives of the pool.

Q: Is it possible to set a slab size?

A: No.

Q: What is a space?

A: Space is a virtual disk created on the physical drives of a pool.

Q: What is thin provisioning?

A: Thin provisioning is a new feature implemented in Storage Spaces using which you can create virtual logical disks larger than a real physical capacity you have. With thin provisioning enabled, disk space is allocated as needed rather than all at once. This makes it easy to add physical drives on an “as needed” basis, which is not possible or difficult for a hardware RAID or dynamic disks (Windows software RAID).

However, if you plan to use thin provisioning you should keep in mind limitations imposed by the number of columns in a thin-provisioned space. Often, comes up as an unpleasant surprise the requirement to add several drives at the same time rather than just one drive.

Q: Thin provisioning - a characteristic of a pool or a space?

A: Thin provisioning relates to a space. A pool may contain both thin-provisioned and fixed-provisioned spaces.

Q: Is it possible to change the type of provisioning after a pool has been created?

A: No.

Q: What is a thin-provisioned space?

A: Thin-provisioned space is a virtual disk, which appears larger than a real physical capacity allocated for it. For such spaces, disk capacity is allocated as needed. 

Q: What is a fixed space (with fixed provisioning)?

A: Fixed space is a space where physical disk capacity is provided for all the claimed space capacity at the moment when the space is created. For a fixed space, it is guaranteed that it is possible to use all the capacity without adding new physical drives.

Q: How many versions of Storage Spaces exist at the moment?

A: Officially, there are two versions of Storage Spaces. The first version is used in Windows Server 2012 and Windows 8, the second version is used in Windows Server 2012 R2. However, we should also mention a “zeroth” version of Storage Spaces that existed in the earlier beta versions of Windows 8. This “zeroth” version is not fully compatible with the subsequent versions, and when you transfer a pool from the early beta versions of Windows to the RTM versions, parity spaces are lost.

Q: How to create a pool and a space? What tool and commands should I use?

A: In Windows 8 you should use the Control Panel. In Windows Server 2012 and Windows Server 2012 R2 you can use either Server Manager (section “File and Storage Services”) or PowerShell. To get a complete list of PowerShell commands associated with Storage Spaces, type in

Get-Command –Module Storage

The complete list of commands is too large to be shown here.

Q: Is it possible to add drives to the pool after it has been created?

A: Yes. However, it may happen that added physical capacity cannot be used. It depends on the number of columns in the space. The default number of columns is determined by the type of space and the number of physical drives initially available (when the space was created).

Q: What types of spaces are possible? What is a simple space? What is a regular mirror space? What is a three way mirror space? What is a parity space? Is there a correlation between Storage Spaces layouts and RAIDs?

A: There are the following layouts in Storage Spaces:

Simple – with no redundancy. This layout is similar to a RAID 0 and the data is distributed across all the physical drives. If one of the drives fails, it is impossible to recover data.

Mirror layout is similar to a RAID1. The data is duplicated among the physical drives that allows surviving one drive failure. Depending on the number of physical drives available when the space is created, variations similar to a RAID10 or RAID1E can be used.

Three-way mirror differs from a regular mirror in that three copies of data are stored, so it is possible to survive simultaneous failure of two drives.

Parity layout is similar to a RAID5. In addition to the user data, parity data is stored as well that allows to survive one member failure. 

In Windows Server 2012 R2, it is possible to use dual parity, which is equivalent to a traditional RAID6 layout.

For simple spaces, Storage Spaces uses RAID0 technology, for mirror spaces - RAID1, RAID1E, or RAID 10 are used, parity spaces utilize RAID5 techniques. If you create a space using PowerShell, you can set the block (stripe) size for a space. When creating a space from Control Panel, you are not allowed to set the block size – it is just fixed at 256 KB. The number of columns in the layout is limited by the number of physical drives available at the time when space was created, but never exceeds 8.

Once layout is set, Storage Spaces cuts each RAID column into 256 MB pieces called slabs. Then these slabs are placed on the physical drives in an arbitrary manner while adhering to the typical RAID rules. For example, two identical (mirror) RAID1 slabs cannot be placed on the same physical drive.

Q: Is it possible to change the type of an existing space?

A: No.

Q: Is it possible to create different spaces (for example, mirror and simple) on the same pool?

A: Yes.

Q: How to determine the number of columns and what is the maximum number of columns in a space?

A: There is a minimum limit on the number of columns, imposed by the type of space (RAID level), namely for a simple space - one, mirror - two, parity and three-way mirror – three. A regular mirror always creates an even number of columns and a three-way mirror creates either three or six columns. The number of columns in a space of any type does not exceed eight.

Q: How is disk space used and how is it related to the number of columns?

A: For a fixed-provisioned space, a disk capacity is allocated as soon as the fixed-provisioned space is created. Capacity of a fixed space can be fully used without adding new drives to the pool and no matter what other spaces are created or used in the same pool.

A thin-provisioned space is much more complex. When you create a thin-provisioned space, the number of columns for it is defined once and for all. Then, when it is needed to allocate physical free space you need N * 256 MB of free space. Thus, it is required to allocate 256 MB on each of the N different physical drives, where N is the number of columns in the space. No rebalancing is provided and the number of columns is never changed.

For example, initially you have two drives in the pool with the size of 1TB and 2TB on which you create a two-column simple space. In this case, the simple space can only use 2 TB, despite the fact that there are 3 TB of physical capacity available. If you want to avoid this, you can create a single-drive pool with a single-column simple space on it, and only then add the second drive. Thus, you ensure that there is only one column in a simple space. As a result, you can use all the capacity, but at the same time you lose the advantage of RAID0 in reading and writing.

Q: What filesystems can be used on spaces?

A: Microsoft recommends using NTFS or ReFS. However, a space can be formatted to any of the supported Windows filesystems, for example, you can use FAT or exFAT. In fact, you can even install the HFS driver from Paragon and use it with Storage Spaces.

Q: What is the recommended filesystem to be used with Storage Spaces and why?

A: For simple spaces and with a small number of files, choose NTFS since it will be faster and will have less overhead. For mirrored and parity spaces, choose ReFS because it better utilizes Storage Spaces features. When using a large number of files, choose ReFS since it is faster.

Q: How many drives can be added to a Storage Spaces pool? What is the maximum pool size? How many spaces can be created on the same pool?

A: Recommended limit is up to 128 spaces on the same pool, no more than 160 drives in the pool, and no more than 480 TB total pool capacity.

Q: Is it possible to include the same drive to the different pools?

A: No. If you really need to work around this limitation, you can create several virtual disks (VHDX) on the same physical drive using Disk Management and then add these virtual disks to the different pools. However, you should keep in mind that such configurations are not supported by Microsoft.

Q: What is the minimum number of drives required to create a pool? How many drives are required for a three-way mirror space? How many drives are required for a parity space?

A: You can create a pool having only one physical drive, but on such a pool it is possible to create only simple spaces.

A two-way (regular) mirror requires two drives.

For a three-way mirror space, you need to provide at least five hard drives.

A parity space, similar to a RAID5, requires three drives minimum.

Q: Can I combine different drives (by vendor, performance, size) in one pool and what about performance?

A: Yes, you can combine different drives. However, it is difficult to predict performance especially if you use drives with different speeds. It is impossible to make any predictions for the configurations mixing SSD and rotational drives, 7200 and 5400 RPM drives, or USB and SATA connections. In addition, as the drive is filled with data, its performance decreases, that further complicates the predictions.

Q: What happens when a thin-provisioned space lacks disk space? How many drives you need to add when a thin-provisioned space runs out of capacity?

A: When a thin-provisioned space runs low on physical capacity (when there is about 30% of free space), Storage Spaces sends a warning message via Action Center and the Storage Spaces Control Panel informing you that it is needed to add drives to a pool - “No capacity; add N drives”. The number of drives that you need to add depends on the number of columns in the space that runs out of capacity.

Q: What happens if one of the drives drops out of the pool? How to find out about the failure?

A: For a simple layout, failure of one drive leads to immediate data loss. In all other cases (mirror, three–way mirror, and parity), the user data is still OK. If there is a hotspare drive in a pool or drives in a pool have enough free space, Storage Spaces restores redundancy automatically by creating new copies of data on a free disk space.

In any case, Action Center shows a message stating that something is wrong with the pool (an error message “Check Storage Spaces for issues”).

Q: What are the drives with an “unrecognized configuration”?

A: The drives with an unrecognized configuration are the drives that formerly belonged to a particular pool of Storage Spaces, but now these drives cannot be identified as belonging to the existing pools due to some problems with the metadata records.

Q: What are the additional features in Windows Server 2012 R2?


  1. Tiered storage, including write-back cache that allows using SSDs to improve overall performance.

  2. Dual parity, a RAID 6 equivalent.

  3. Improved rebuild procedures.

Q: Are the first and the second version of Storage Spaces compatible with each other?

A: You can bring a pool of an older version of Storage Spaces to a newer version, but not vice versa.

Q: Is it possible to transfer Storage Spaces pools between different Windows installations?

A: Yes, if you take into account these conditions:

  • The new system supports the required version of Storage Spaces. For example, you cannot transfer a Storage Spaces pool to Windows XP. Also you cannot transfer a pool created in Windows 2012 Server R2 to Windows 2012 Server.

  • The new system supports filesystems used on the spaces of the pool. If there is a space with ReFS, you cannot connect it to a Windows 8 PC, since it just cannot read this filesystem.

  • You have to transfer enough drives simultaneously. You can leave some drives behind (for example, for a parity space it is acceptable not to include one of the drives); however this is not a recommended approach. It is always recommended to transfer all the drives if possible.

Q: What Windows versions support Storage Spaces?


  • Windows 8, where you can format a space to any Windows filesystem except ReFS since Windows 8 (not Server) does not support ReFS at all.

  • Windows Server 2012 with the first version of Storage Spaces.

  • Windows Server 2012 R2, using the second version of Storage Spaces with additional features.

Q: What is the performance of a space compared to the RAID arrays?

A: As for read speed, it is all about the same as in corresponding RAID, taking into account the number of columns, the performance of a single drive, and the connection type.

If we compare write speed, the performance of a simple space is similar to that of a RAID0. Write speed of a mirror space is about the same as for a RAID10 or RAID 1E, again considering the number of columns and drives.

A parity space in Storage Spaces is noticeably slower than a hardware RAID5 for large operations (when it is needed to write large amounts of data). This is because Storage Spaces uses journaling to avoid RAID5 write hole. When you copy small amounts of data (up to 500MB), the cache compensates the speed degradation. For large amounts of data, write speed drops and is not stable since once the cache and the log are full, write operations cannot be deferred any longer. Thus, write speed drops until the cache is flushed and the log is committed.

Also pay attention to the following:

In a hardware RAID, performance of individual drives is significantly less than the throughput of the bus and so the performance adds up.

In Storage Spaces, people often connect drives via USB. The throughput of USB limits total performance. Say, for USB 2.0, the bus throughput is less than performance of a modern drive.

Q: What are the signs that something is wrong with a pool - the error messages and so on? When does the space go offline?

A: The first sign of a severe problem with the pool is that you cannot read the data or the drive letters of respective spaces have been lost. The reasons of this may be less ominous, for example:

  1. Some drives dropped out, for example due to the loss of USB electrical contact.

  2. There is no more free space available on a thin-provisioned space and this space went offline.

or more ominous:

  1. The number of physically damaged drives in the pool exceeds the number covered by redundancy.

  2. The Storage Spaces metadata is damaged.

In any case, you should check Action Center and Control Panel of Storage Spaces for error messages. Typically, you can find enough information in there to determine what happened.

If the problem is not serious enough to send the space offline, but it still requires your attention, a message appears in Action Center. Usually in this case you need either to replace a drive or add new drives since free space is low. The warning message informing user that the free space is low is shown when there is about 30% of free disk space.


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