Administrator’s Command Line Guide

Chapter 6. Maximizing Storage Cluster Performance
The typical reasons of poor performance with 4KB sector HDDs are:
1. Host OS file system unaligned on the 4KB boundary. The make-cs command tries to detect and report
such issues to the administrator in advance, but be aware that the fdisk utility is not recommended for
partitioning HDDs. You should use parted instead.
2. Unaligned writes (e.g., 1KB) performed by guest OS. Many legacy operating systems, like Microsoft
Windows XP and Windows Server 2003 or Red Hat Enterprise Linux 5.x, have unaligned partitions by
default and generate unaligned I/O patterns which are quite slow on both Acronis Storage and actual
HDDs with 4KB sectors. If you plan running such legacy operating systems, consider the following:
• Using smaller HDDs with 512-byte sectors, or use SSD journaling for CS services which mitigates
the issue to some extent.
• Aligning OS partitions properly.
You can check for unaligned write operations in the storage cluster as follows:
1. Run the vstorage top or stat command. For example:
# vstorage -c stor1 top
2. Press i to display the RMW and JRMW columns in the CS part of the top output.
3. Check the RMW or JRMW counters, which are explained below.
• When SSD journaling is used, the RMW counter shows the number of requests which lead to
Read-Modify-Write cycles, while the JRMW counter shows the number of Read-Modify-Write cycles
mitigated by the use of SSD journals.
• When SSD journaling is not used, the JRMW counter shows the number of unaligned requests
which potentially generate Read-Modify-Write cycles on the HDD in question.
6.6 Disabling Inter-Tier Data Allocation
If a storage tier runs out of free space, Acronis Storage will attempt to temporarily use the space of the lower
tiers down to the lowest. If the lowest tier also becomes full, Acronis Storage will attempt to use a higher one.
If you add more storage to the original tier later, the data, temporarily stored elsewhere, will be moved to the
tier where it should have been stored originally.
Mixing tier workloads is not recommended as it may decrease cluster performance. To prevent this, you can
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