Stripe size (strip size) basics for hardware RAID

When a RAID controller stripes data across drives, it writes a fixed-size chunk to one drive, then the next chunk to the next drive, and so on. The size of that chunk is the strip size (Dell calls it the strip element size, HPE's SSA CLI calls it stripe size). It is chosen when the volume is created and affects performance, background scans and rebuilds. This guide explains the terms, the sizes each Dell and HPE controller offers, how to choose, and what changing it involves.

Strip, stripe and stripe element

• Dell: the PERC guides describe disk striping as dividing each drive's space into stripes, interleaved in sequence. "The part of the stripe on a single physical disk is called a stripe element." The setting in the PERC configuration utility is Strip Element Size.
• HPE: the Smart Array and MR guides call the unit of data stored on each drive a "strip" and give a range of 64 KiB to 1 MiB. The SSA CLI uses the parameter ss= and the term stripe size for the same value.

A full stripe is one strip on every drive that holds data. For example, RAID 5 on six drives with a 256 KB strip holds 5 × 256 KB = 1,280 KB of data per full stripe, plus one parity strip. HPE explains that when a write covers a full stripe, the controller computes the new parity from the new data without reading old data, so there is almost no parity write penalty.

Sizes each controller offers

The SSA CLI syntax accepts ss=8|16|32|64|128|256|512|1024|default|?, but a given controller shows only the values it supports. HPE's example query returned 8 to 256, with 256 marked as the default for that logical drive.

ssacli ctrl slot=0 ld 1 modify raid=0 ss=?

HPE notes that you cannot query RAID level and stripe size in the same command, and that if you leave out the stripe size the CLI uses the default for the RAID level you specify.

How to choose a strip size

• Match the application. HPE: the best performance and drive longevity come from aligning and sizing the strip to the application's I/O request size.
• Small strips cost background time. HPE: the smaller the strip (64 KiB or less), the longer background parity scans and rebuilds take and the more they affect host I/O. HPE's MR guide adds that multiple stripes can be checked or rebuilt concurrently.
• SSDs on Dell PERC: 64 KB. Dell's PERC 11 guide says to create SSD virtual disks with a 64 KB strip for optimal SSD performance with FastPath, and only I/O smaller than the stripe size is eligible for FastPath. PERC 12 allows only 64 KB on SSDs anyway. See SSD vs HDD arrays.
• When unsure, keep the default. Dell's PERC 11 default is 256 KB, and the HPE CLI applies the RAID level's default when you do not specify one.

Step 1 — Check the current strip size

• Dell PERC 11 (HII): Main Menu > Virtual Disk Management, click the virtual disk, then Advanced.... The Strip element size field shows the value.
• HPE MR Gen11 (UEFI): System Configuration > HPE MRXXX Gen11 > Main Menu > Logical Drive Management, select the logical drive, then Advanced... and read Strip size.
• HPE Smart Array (SSA CLI): show the controller configuration in detail:

ssacli ctrl slot=0 show config detail

Step 2 — Change the strip size

Dell PERC

The PERC 11 guide documents online reconfiguration only as capacity expansion and RAID level migration, and the PERC 12 unified guide states that RAID level migration is not supported on H965 and H365 series controllers. Neither guide documents an online strip-size change, so plan to back up, delete the virtual disk, create it again with the new Strip Element Size, and restore. Creation steps: create a virtual disk on Dell PERC.

HPE Smart Array SR

Smart Array controllers support a Migrate strip size transformation on a live logical drive. HPE's notes:

• Moving to a larger strip size may need unused space in the array, because some larger stripes will be filled inefficiently.
• The SSA CLI guide says an expansion, extension or migration takes about 15 minutes per gigabyte, and only one can run at a time on a controller.
• Transformation priority (High, Medium, Low) trades speed against impact on normal I/O.

ssacli ctrl slot=4 ld 2 modify ss=16
ssacli ctrl slot=2 ld 3 modify raid=5 ss=16

These examples are from HPE's guide; the second changes RAID level and stripe size together. More commands: HPE ssacli RAID commands.

HPE MR Gen11

The MR Gen11 guide documents RAID level migration through Modify Array in HPE MR Storage Administrator, but not a strip-size migration. Unless your MR software documentation says otherwise, treat a strip-size change as a re-create with backup and restore.

Frequently asked questions

Does strip size matter on SSD arrays?

Yes, on Dell PERC it decides whether FastPath can help: Dell recommends 64 KB for SSD virtual disks, and PERC 12 enforces it. On HPE, the general rule of matching the strip to the application I/O size applies to SSDs too.

Can I choose different strip sizes for different volumes?

The strip size is a property of each virtual disk or logical drive: Dell shows it per virtual disk and HPE MR per logical drive, and SSA CLI changes it per logical drive (ld 2 modify ss=).

Checklist

• Know the dominant I/O size of the workload before creating the volume.
• Check the strip sizes your controller and drive type allow.
• SSD on Dell PERC: 64 KB. Otherwise keep the default unless you have a measured reason.
• Avoid very small strips on large parity arrays because scans and rebuilds take longer.
• Record the strip size with the server's configuration notes.
• Back up before any change; on Dell PERC and HPE MR plan a re-create, on HPE Smart Array allow time for migration.

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