# BTCPay Synchronization Issues

This document covers the most common questions and issues that may occur during BTCPay sync.

# Why does BTCPay sync?

After deployment, your BTCPay Server needs to sync the entire blockchain and validate all the consensus rules. Depending on your machine specifications, bandwidth and number of altcoins you added, this process may take between 1-5 days.

It may seem tedious, but it's a critical step of running your own full node and not having to trust or rely on anyone. Your node will not only download ~300GB of data (less if you're using a pruned node) but also validate all the rules of the consensus. You can find more information about the importance of blockchain synchronization in this video.

# Can I skip the synchronization?

You can't skip synchronization, but you can drastically decrease the time it takes. If you're comfortable with using the command line, you can use FastSync to synchronize your node faster. Be sure to read this FastSync document to understand the potential trust issues involved with this feature.

To use FastSync, make sure your deployment has a pruning option enabled by using an opt-save-storage environment variable, otherwise bitcoind will not be able to sync. First step is to ssh into your BTCPayServer instance and run the following commands:

sudo su -
cd $BTCPAY_BASE_DIRECTORY/btcpayserver-docker/
cd contrib/FastSync
# Once FastSync has completed

After FastSync is complete and you have brought back up your instance, refresh your BTCPay domain and wait for remaining blockchain synchronization. You can also follow this video.

If your FastSync returns You need to delete your Bitcoin Core wallet after you load the uxto set, or you find this error: Last wallet synchronisation goes beyond pruned data, see the cause of BTCPay Server keeps showing that my node is always starting.

# How do I know that BTCPay synced completely?

When you do not see a pop-up message in the bottom right corner, which shows the sync progress, that means that your server is fully synced and you can begin using it.

# BTCPay Server takes forever to synchronize

Synchronizing a Full Bitcoin node should take between 1 and 5 days. It should sync quickly at first and more slowly at the end.

If the node appears to not be syncing, verify:

  • Not enough CPU
  • Using swap memory

# Cause 1: Not enough CPU

We recommend 2 CPU while synchronizing; however some hosting providers throttle your CPU if you use too much.

Check with

sudo su -
docker stats

If you see more than 100% CPU usage, while being very slow to sync:

8e7ac41e6e2a        btcpayserver_bitcoind               100%               560.5MiB / 3.853GiB   14.20%              4.17

Then you need to scale up your machine specification.

If you see very low CPU usage (less than 10%) during synchronization:

8e7ac41e6e2a        btcpayserver_bitcoind               10%               560.5MiB / 3.853GiB   14.20%              4.17

Your hosting provider might throttle your CPU. Please make sure your host supports the high use of CPU for an extended period.

If they don't allow it, shut down your server until they stop throttling you. Then you can limit the CPU via docker, and restart the server:

docker update btcpayserver_bitcoind --cpus ".8"

# Cause 2: Using swap memory

If you are synching and don't have enough memory, your server may use swap memory to continue operating:

sudo su -
df -h

If you see swap memory usage:

              total        used        free      shared  buff/cache   available
Mem:           2.0G        2.0G        0M         66M        0G        0M
Swap:          1.0G        200M      800M

Then it means you need to scale up your server by adding more memory.

# BTCPay Server keeps showing that my node is always starting

Possible cause:

  • Your bitcoin data directory is corrupted
  • You do not have enough RAM
  • You do not have enough storage
  • Your last wallet synchronisation goes beyond pruned data

# Cause 1: Your bitcoin data directory is corrupted

Check the logs of your node:

sudo su -
docker logs --tail 10 btcpayserver_bitcoind

If you see:

Please restart with -reindex or -reindex-chainstate to recover.

Then your bitcoin data directory has been corrupted. It may be physical damage or failure of the hard drive. To reindex your node:

# Delete 'blocks' and 'chainstate' folders
rm -rf /var/lib/docker/volumes/generated_bitcoin_datadir/_data/blocks
rm -rf /var/lib/docker/volumes/generated_bitcoin_datadir/_data/chainstate

# Cause 2: You do not have enough RAM

Check your RAM:

sudo su -
free -h

If you see that you have no free or very little available memory:

              total        used        free      shared  buff/cache   available
Mem:           2.0G        2.0G        0M         66M        0G        0M
Swap:            0B          0B          0B

Then you need more memory. If you have already synched your node, you can add some swap memory. If you haven't, your server specs are too limited.

If you have already synched, you can add 2G of swap memory with:

fallocate -l 2G /mnt/swapfile
chmod 600 /mnt/swapfile
mkswap /mnt/swapfile
swapon /mnt/swapfile
echo "/mnt/swapfile   none    swap    sw    0   0" >> /etc/fstab

# Cause 3: You do not have enough storage

Check the storage of your machine:

sudo su -
df -h

If you see you don't have any storage left (/dev/sda1 in my case)

Filesystem      Size  Used Avail Use% Mounted on
udev            2.0G     0  2.0G   0% /dev
tmpfs           395M   41M  354M  11% /run
/dev/sda1       125G  125G  0G   100% /
tmpfs           2.0G     0  2.0G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           2.0G     0  2.0G   0% /sys/fs/cgroup
/dev/sdb1       7.8G   18M  7.4G   1% /mnt

Choose the docker fragment for the amount of storage you aim to keep.

Then prune your node.

# Cause 4: Your last wallet synchronisation goes beyond pruned data

This can happen if you use FastSync or import an already synched blockchain. It means that the bitcoin core wallet needs to be removed because it was created before the utxoset, likely because BTCPay Server started without the utxoset at the first boot. To verify this case, check the bitcoind log for this:

Error: Prune: last wallet synchronisation goes beyond pruned data. You need to -reindex (download the whole blockchain again in case of pruned node)

If you see this error and agree to remove the wallet to finish syncing, use docker volume rm generated_bitcoin_wallet_datadir after you run btcpay-down.sh and before you run btcpay-up.sh WARNING: Do not delete this wallet if you have any funds on it.

# I'm running a full node and have a synched blockchain, can BTCPay use it so that it doesn't have to do a full sync?

Yes you can! However, before you do that, you'll want to stop bitcoind from updating docker's volume for it, as that job will be taken over by BTCPay Server.

If you want to run BTCPay Server inside a docker-compose, and that you have the data directory (.bitcoin) of a fully synched node on your docker host, then you can reuse it easily for BTCPay Server.

To do that, follow the following steps :

  • Do the normal setup according to this instruction. Note the opt-save-storage environment variable, which is used to enable various pruning levels. If you do not want to prune your exiting data directory, then omit the following line in your BTCPay docker deployment: export BTCPAYGEN_ADDITIONAL_FRAGMENTS="opt-save-storage-s".
  • Once btcpay-setup.sh is over, turn down the docker compose with btcpay-down.sh.
  • Login as root with sudo su -.
  • Open the docker's volume for bitcoind : cd /var/lib/docker/volumes/generated_bitcoin_datadir/, and check its content with ls -la. You should see only one directory named _data.
  • Now remove the _datadirectory : rm -r _data. If for any reason you want to keep this directory and its content you can also rename it instead : mv _data/ _data.old/
  • Now create a symbolic link between /var/lib/docker/volumes/generated_bitcoin_datadir/_data and your data directory (.bitcoin) on your host: ln -s path/to/.bitcoin /var/lib/docker/volumes/generated_bitcoin_datadir/_data
  • Check that the link has been done with a ls -la
  • Start your docker-compose again with btcpay-up.sh

Your BTCPay Server should now be fully synched.

If after this BTCPay Server keeps showing that your node is always starting, see the cause of BTCPay Server keeps showing that my node is always starting.