Flashes (Alerts)
Abstract
Misaligned LUNs with VMware ESX Server .vmdk virtual disk files (VMFS or Guest OS) can cause poor filer read and write i/o performance due to additional overhead from partial writes.
Content
Partial writes can occur when using VMFS filesystem or NFS based VMDKs on ESX.
This issue is not unique to N series storage system. Any storage vendor or host platform may exhibit this problem. You can determine if partial writes are occurring by looking at the 'wp.partial_write' setting in perfstat output. 'wp.partial_write' is a block counter of misaligned I/O. In Data ONTAP 7.2.1 and later, 'read_/write_align_histo.XX' and 'read_ /write_partial_blocks.XX / are also available in the 'stats stop -I perfstat_lun' section of a perfstat output.
Note: Raw Device Mapping (RDM) LUNs are not affected by partial writes as long as the LUN type is set to the RDM OS type.
Solution
The blocks for both the VMFS filesystem and the Guest OS filesystem on the .vmdk must be aligned with LUNs to prevent partial writes. The LUN alignment is a recommended best practice.
See the following technical report for a description of the issue, impact and solutions:
Aligning file systems in virtual environments with IBM System Storage N series Best practices
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Document Information
Modified date:
25 September 2022
UID
ssg1S1003518