Swap Space allocated for BOSH-Deployed VMs

Products

• Operations Manager

Issue/Introduction

How much swap space is allocated out of the total ephemeral disk space in a BOSH-deployed virtual machine (VM)?

Resolution

BOSH creates 2 partitions on the ephemeral disk /dev/sdb:

1. /dev/sdb1
   • Used for swap.
2. /dev/sdb2
   • Used for ephemeral storage.
   • Mounted to /var/vcap/data.

BOSH will create the /dev/sdb1 swap partition to be equal to one of the following, whichever is smaller:

• RAM
• 1/2 of ephemeral disk

This means the swap partition can consume up to 1/2 of the ephemeral disk for instance types with large amounts of memory relative to the disk size.

Some examples with different RAM and ephemeral disk sizes:

RAM	Ephemeral Disk	Swap (sdb1)	/var/vcap/data/ (sdb2)	Notes
1 GB	2 GB	1 GB	1 GB	Set to RAM
512 MB	1 GB	512 MB	1.5 GB	Set to RAM
2 GB	4 GB	2 GB	2 GB	Set to RAM
2 GB	16 GB	2 GB	14 GB	Set to RAM
4 GB	8 GB	4 GB	4 GB	Set to RAM
8 GB	8 GB	4 GB	4 GB	Set to 1/2 ephemeral disk
4 GB	32 GB	4 GB	28 GB	Set to RAM
2 GB	8 GB	2 GB	6 GB	Set to RAM
8 GB	16 GB	8 GB	8 GB	Set to RAM
16 GB	16 GB	8 GB	8 GB	Set to 1/2 ephemeral disk
8 GB	64 GB	8 GB	56 GB	Set to RAM
4 GB	16 GB	4 GB	12 GB	Set to RAM
16 GB	32 GB	16 GB	16 GB	Set to RAM
32 GB	32 GB	16 GB	16 GB	Set to 1/2 ephemeral disk
16 GB	128 GB	16 GB	112 GB	Set to RAM
8 GB	32 GB	8 GB	24 GB	Set to RAM
32 GB	64 GB	32 GB	32 GB	Set to RAM
64 GB	64 GB	32 GB	32 GB	Set to 1/2 ephemeral disk
32 GB	256 GB	32 GB	224 GB	Set to RAM
16 GB	64 GB	16 GB	48 GB	Set to RAM