This KB provides the solution to fix ISSU upgrade failure with the error: node1-LINUX: Available space: 1926464 require: 1048576 * 2 

You will receive the following error while upgrade the cluster using ISSU method:

error: node1-LINUX: Available space: 1926464 require: 1048576 * 2
ISSU: Pre-check failure
node0:
JSRPD exited in-service-upgrade window
node1:
JSRPD exited in-service-upgrade window
Chassis ISSU Aborted
node0:
Chassis ISSU Aborted
node1:
Chassis ISSU Aborted
ISSU: IDLE
[Aug 11 05:43:35]: ISSU aborted and exiting ISSU window.

During the ISSU upgrade if the above error is received, check and clear the storage on both the nodes. Below are the command to check the storage and clear the files. 

SRX> show system storage
SRX> request system storage cleanup dry-run

This will list the files which you can delete from SRX to increase the storage.