The Impact of Logical and Physical Fragmentation in a Virtual Environment Presented by Raxco Software, Inc. October 29, 2009
What The Experts Say Like physical disk drives, virtual disks can become fragmented. Defragmenting disks rearranges files, programs and unused space on the virtual disk so that programs run faster and files open more quickly. VMware Defragmenting Virtual Disks
Experts… Performance in Windows guest operating systems may also be affected by disk fragmentation VMware ACE
Experts… The two biggest reasons I see virtual machines get slower over time is due to (1) incorrect virtual machine additions installed and (2) disk fragmentation. Andrew Connell Former MS Share Point Developer Microsoft MVP-Content Management Server
What is Fragmentation?
Sample Fragmentation Problems minutes to launch Notes on laptops Users locked out of servers due to insufficient contiguous free space Servers wont boot Application hangs and crashes System errors due to metadata fragmentation Video/audio flickers Prolonged backup
Cost of Fragmentation Productivity Support Unnecessary hardware Longer backups Opportunity
The Fragmentation Issue Logical fragmentation -How NTFS sees the disk Physical fragmentation -How the disk controller sees the disk Free space
The Logical Level To format a partition Windows needs to know if it is NTFS/FAT, cluster size and disk size Bitmap file is how NTFS views the disk Cannot distinguish between physical disk types MFT is created along with metadata files NTFS
NTFS and the MFT ComponentDescription NTFS Boot Sector The BIOS parameter block that stores information about the layout of the volume and file system structures, plus the boot code that loads Windows Server. Master File Table (MFT)The information necessary to retrieve files from the NTFS partition, such as the attributes of a file. File System DataStores data that is not contained within the Master File Table MFT CopyCopies of the records essential for the recovery of the file system if there is a problem with the original.
File Allocation (Writes) Create MFT record (one or more) name, parent dir, security, allocation, pointers to other records Bitmap accessed to locate free (blocks all) MFT record is updated with content NTFS passes LCNs to disk driver
Anatomy of the MFT
File Access (Reads) Load portion of MFT with correct record via directory Locate file in the MFT Pass starting LCN and run length to disk controller Number of physical disk seeks determined by the number of logical fragments
Wasted Seeks
NTFS Expert Concludes… Free space fragmentation slows down writes Free space fragmentation blocks on write File fragmentation causes needless seeks Slower the disk the greater the bottleneck
The Physical Level NTFS reports VCN the starting LCN and its length to the disk controller Disk controller maps LCN to PCN based on its software/firmware Data written to disk RAID or other disk configurations may physically fragment the file for faster access
Fragmentation and VMs Windows guests have all the same NTFS behavior Resource contention between Windows guests can be an issue Resource contention between guests and the host (Windows-based host) Free space fragmentation not good for dynamic vmdk, vhd
How Defragmentation Helps All defragmentation is at the logical cluster level Defragmentation puts files in strings of contiguous logical clusters ($Bitmap) Controller will work faster Fewer unnecessary seeks to the disk Fastest possible logical and physical I/O
Advantages Centralized management via console Offline defragmentation of system and metadata Consolidates free space Optimized boot files and MFT Requires only 1% free space Fast and light on resources Flexible scheduling Windows Certified Full integration with AD VSS Compatible Integrated with vSphere 4 Virtualization Aware
No Virtual Awareness
Virtualization Awareness
The Choices Disk A Chaos Slow Short Life Cycle Inefficient Messy Disk B Orderly Fast Long Life Cycle Efficient Perfect
For vSphere For Hyper-V For VMware Workstation For Server and Workstation For Exchange