"Unwritable cache data exists for a volume" - this means new writes came to Controller cache but not yet written to backend drives because Vdisk is not available at that point. It could be vdisk was quarantined or offline or QTOF state.
Somewhere you have mentioned that you have cleared Cache data which is a big mistake because this data you lost.
Coming to why vdisk reconstruction failed. This could be due to other drives in same vdisk having hardware errors which is not allowing the vdisk rebuild process to complete. This means MSA controller not able to read from some other drives part of same vdisk to reconstruct RAID data. In RAID 5 more than one drive having issue or failed means it's RAID failed and data recovery difficult.
In future I would always suggest to keep your MSA up to date in terms of firmware with respect to all components like Controllers, IO modules and drives. Also this type of case you should always take help from HPE support.
Right now I would suggest to figure out other faulty drive and delete the vdisk. Then replace all faulty drives together. Create new vdisk and volumes as per your requirement. Then restore data from backup.
Hope this helps!
Regards
Subhajit
I am an HPE employee
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