Multipartition Volume Management

Multipartition Volume Management

FtDisk and DMIO are responsible for presenting volumes that file system drivers manage and for mapping I/O directed at volumes to the underlying partitions that they're made of. For simple volumes, this process is straightforward: the volume manager ensures that volume-relative offsets are translated to disk-relative offsets by adding the volume-relative offset to the volume's starting disk offset.

Multipartition volumes are more complex because the partitions that make up a volume can be located on discontiguous partitions or even on different disks. Some types of multipartition volumes use data redundancy, so they require more involved volume-to-disk-offset translation. Thus, FtDisk and DMIO must process all I/O requests aimed at the multipartition volumes they manage by determining which partitions the I/O ultimately affects.

The following types of multipartition volumes are available in Windows 2000:

  • Spanned volumes
  • Mirrored volumes
  • Striped volumes
  • RAID-5 volume

After describing multipartition-volume partition configuration and logical operation for each of the multipartition-volume types, we'll cover the way that the FtDisk and DMIO drivers handle IRPs that a file system driver sends to multipartition volumes. The term volume manager is used to represent both FtDisk and DMIO throughout the explanation of multipartition volumes, because both FtDisk and DMIO support the same multipartition-volume types.

Spanned Volumes

A spanned volume is a single logical volume composed of a maximum of 32 free partitions on one or more disks. The Windows 2000 Disk Management MMC snap-in combines the partitions into a spanned volume, which can then be formatted for any of the Windows 2000-supported file systems. Figure shows a 100-MB spanned volume identified by drive letter D: that has been created from the last third of the first disk and the first third of the second. Spanned volumes were called "volume sets" in Windows NT 4.

Click to view at full size.

Figure Spanned volume

A spanned volume is useful for consolidating small areas of free disk space into one larger volume or for creating a single, large volume out of two or more small disks. If the spanned volume has been formatted for NTFS, it can be extended to include additional free areas or additional disks without affecting the data already stored on the volume. This extensibility is one of the biggest benefits of describing all data on an NTFS volume as a file. NTFS can dynamically increase the size of a logical volume because the bitmap that records the allocation status of the volume is just another file—the bitmap file. The bitmap file can be extended to include any space added to the volume. Dynamically extending a FAT volume, on the other hand, would require the FAT itself to be extended, which would dislocate everything else on the disk.

A volume manager hides the physical configuration of disks from the file systems installed on Windows 2000. NTFS, for example, views volume D: in Figure as an ordinary 100-MB volume. NTFS consults its bitmap to determine what space in the volume is free for allocation. It then calls the volume manager to read or write data beginning at a particular byte offset on the volume. The volume manager views the physical sectors in the spanned volume as numbered sequentially from the first free area on the first disk to the last free area on the last disk. It determines which physical sector on which disk corresponds to the supplied byte offset.

Striped Volumes

A striped volume is a series of up to 32 partitions, one partition per disk, that combines into a single logical volume. Striped volumes are also known as RAID level 0 (RAID-0) volumes. Figure shows a striped volume consisting of three partitions, one on each of three disks. (A partition in a striped volume need not span an entire disk; the only restriction is that the partitions on each disk be the same size.)

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Figure Striped volume

To a file system, this striped volume appears to be a single 450-MB volume, but a volume manager optimizes data storage and retrieval times on the striped volume by distributing the volume's data among the physical disks. The volume manager accesses the physical sectors of the disks as if they were numbered sequentially in stripes across the disks, as illustrated in Figure.

Click to view at full size.

Figure Logical numbering of physical sectors on a striped volume

Because each stripe is a relatively narrow 64 KB (a value chosen to prevent individual reads and writes from accessing two disks), the data tends to be distributed evenly among the disks. Stripes thus increase the probability that multiple pending read and write operations will be bound for different disks. And because data on all three disks can be accessed simultaneously, latency time for disk I/O is often reduced, particularly on heavily loaded systems.

Spanned volumes make managing disk volumes more convenient, and striped volumes spread the I/O load over multiple disks. These two volume-management features don't provide the ability to recover data if a disk fails, however. For data recovery, a volume manager implements three redundant storage schemes: mirrored volumes, RAID-5 volumes, and sector sparing. (Sector sparing and NTFS support for sector sparing are described in Chapter 12.) These features are created with the Windows 2000 Disk Management administrative tool.

Mirrored Volumes

In a mirrored volume, the contents of a partition on one disk are duplicated in an equal-sized partition on another disk. Mirrored volumes are sometimes referred to as RAID level 1 (RAID-1). A mirrored volume is shown in Figure.

Figure Mirrored volume

When a program writes to drive C:, the volume manager writes the same data to the same location on the mirror partition. If the first disk or any of the data on its C: partition becomes unreadable because of a hardware or software failure, the volume manager automatically accesses the data from the mirror partition. A mirror volume can be formatted for any of the Windows 2000-supported file systems. The file system drivers remain independent and are not affected by the volume manager's mirroring activity.

Mirrored volumes can aid in I/O throughput on heavily loaded systems. When I/O activity is high, the volume manager balances its read operations between the primary partition and the mirror partition (accounting for the number of unfinished I/O requests pending from each disk). Two read operations can proceed simultaneously and thus theoretically finish in half the time. When a file is modified, both partitions of the mirror set must be written, but disk writes are done asynchronously, so the performance of user-mode programs is generally not affected by the extra disk update.

Mirrored volumes are the only multipartition volume type supported for system and boot volumes. The reason for this is that the Windows 2000 boot code, including the MBR code and Ntldr, don't have the sophistication required to understand multipartition volumes—mirrored volumes are the exception because the boot code treats them as simple volumes, reading from the half of the mirror marked as the boot or system drive in the MS-DOS-style partition table. Because the boot code doesn't modify the disk, it can safely ignore the other half of the mirror.

Watching Mirrored Volume I/O Operations

Using the Windows 2000 Performance tool, you can verify that write operations directed at mirrored volumes copy to both disks that make up the mirror and that read operations, if relatively infrequent, occur primarily from one half of the volume. This experiment requires three hard disks and Windows 2000 Server, Windows 2000 Advanced Server, or Windows 2000 Datacenter Server. If you don't have three disks or a server system, you can skip the experiment setup instructions and view the Performance tool screen shot in this experiment that demonstrates the experiment's results.

Use the Disk Management MMC snap-in to create a mirrored volume. To do this, perform the following steps:

  1. Run Disk Management by starting Computer Management, expanding the Storage tree, and selecting Disk Management (or by inserting Disk Management as a snap-in in an MMC console).
  2. Right-click on an unallocated space of a drive, and select Create Volume.
  3. Follow the instructions in the Create Volume wizard to create a simple volume. (Make sure there's enough room on another disk for a volume of the same size as the one you're creating.)
  4. Right-click on the new volume and select Add Mirror from the context menu.

Once you have a mirrored volume, run the Performance tool and add counters for the PhysicalDisk performance object for both disk instances that contain a partition belonging to the mirror. Select the Disk Reads/Sec and Disk Writes/Sec counters for each instance. Select a large directory from the third disk (the one that isn't part of the mirrored volume) and copy it to the mirrored volume. The Performance tool output window should look something like this as the copy operation progresses:

Click to view at full size.

The top two lines, which overlap throughout the timeline, are the Disk Writes/Sec counters for each disk. The bottom two lines are the Disk Reads/Sec lines. The screen shot reveals that the volume manager (in this case DMIO) is writing the copied file data to both halves of the volume but primarily reading from only one. This read behavior occurs because the number of outstanding I/O operations during the copy didn't warrant that the volume manager perform more aggressive read-operation load balancing.

RAID-5 Volumes

A RAID-5 volume is a fault tolerant variant of a regular striped volume. RAID5 volumes implement RAID level 5. They are also known as striped volume with parity because they are based on the striping approach taken by striped volumes. Fault tolerance is achieved by reserving the equivalent of one disk for storing parity for each stripe. Figure is a visual representation of a RAID5 volume.

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Figure RAID-5 volume

In Figure, the parity for stripe 1 is stored on disk 1. It contains a byte-for-byte logical sum (XOR) of the first stripe on disks 2 and 3. The parity for stripe 2 is stored on disk 2, and the parity for stripe 3 is stored on disk 3. Rotating the parity across the disks in this way is an I/O optimization technique. Each time data is written to a disk, the parity bytes corresponding to the modified bytes must be recalculated and rewritten. If the parity were always written to the same disk, that disk would be busy continually and could become an I/O bottleneck.

Recovering a failed disk in a RAID-5 volume relies on a simple arithmetic principle: in an equation with n variables, if you know the value of n - 1 of the variables, you can determine the value of the missing variable by subtraction. For example, in the equation x + y = z, where z represents the parity stripe, the volume manager computes z - y to determine the contents of x; to find y, it computes z - x. The volume manager uses similar logic to recover lost data. If a disk in a RAID-5 volume fails or if data on one disk becomes unreadable, the volume manager reconstructs the missing data by using the XOR operation (bitwise logical addition).

If disk 1 in Figure fails, the contents of its stripes 2 and 5 are calculated by XORing the corresponding stripes of disk 3 with the parity stripes on disk 2. The contents of stripes 3 and 6 on disk 1 are similarly determined by XORing the corresponding stripes of disk 2 with the parity stripes on disk 3. At least three disks (or rather, three same-sized partitions on three disks) are required to create a RAID-5 volume.

Volume I/O Operations

File system drivers manage data stored on volumes but rely on volume managers to interact with storage drivers to transfer data to and from the disk or disks on which a volume resides. File system drivers obtain references to a volume manager's volume objects through the mount process (described later in this chapter) and then send the volume manager requests via the volume objects. Applications can also send the volume manager requests, bypassing file system drivers, when they want to directly manipulate a volume's data. File-undelete programs are an example of applications that do this, and so is the DiskProbe utility that's part of the Windows 2000 resource kits.

Whenever a file system driver or application sends an I/O request to a device object that represents a volume, the Windows 2000 I/O manager routes the request (which comes in an IRP—a self-contained package) to the volume manager that created the target device object. Thus, if an application wants to read the boot sector of the second volume on the system, it opens the device object \Device\HarddiskVolume2 and then sends the object a request to read 512 bytes starting at offset zero on the device. The I/O manager sends the application's request in the form of an IRP to the volume manager that owns the device object, notifying it that the IRP is directed at the HarddiskVolume2 device.

Because partitions are logical conveniences that Windows 2000 uses to represent contiguous areas on a physical disk, the volume manager must translate offsets that are relative to a partition to offsets that are relative to the beginning of a disk. If partition 2 begins 3449 sectors into the disk, the volume manager would adjust the IRP's parameters to designate an offset with that value before passing the request to the disk class driver. The disk class driver uses a miniport driver to carry out physical disk I/O and read requested data into an application buffer designated in the IRP.

Some examples of a volume manager's operations will help clarify its role when it handles requests aimed at multipartition volumes. If a striped volume consists of two partitions, partition 1 and partition 2, that are represented by the device object \Device\HarddiskDmVolumes\PhysicalDmVolumes\BlockVolume3 , as Figure shows, and an administrator has assigned drive letter D to the stripe, the I/O manager defines the link \??\D: to reference \Device\HarddiskDmVolumes\ComputerNameDg0\Volume3, where ComputerName is the name of the computer. Recall from earlier that this link is also a symbolic link, and it points to the corresponding volume device object in the PhysicalDmVolumes directory (in this case, BlockVolume3). The DMIO device object intercepts file system disk I/O aimed at \Device\HarddiskDmVolumes\PhysicalDmVolumes\BlockVolume3, and the DMIO driver adjusts the request before passing it to the Disk class driver. The adjustment DMIO makes configures the request to refer to the correct offset of the request's target stripe on either partition 1 or partition 2. If the I/O spans both partitions of the volume, DMIO must issue two subsidiary I/O requests, one aimed at each disk.

Click to view at full size.

Figure DMIO I/O operations

In the case of writes to a mirrored volume, DMIO splits each request so that each half of the mirror receives the write operation. For mirrored reads, DMIO performs a read from half of a mirror, relying on the other half when a read operation fails.

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